Nephrology Practice Questions — USMLE Step 2 CK — Page 2

Question 21Question →

An obtunded 68-year-old man is brought to the emergency department by paramedics following a two-day history of worsening shortness of breath, productive cough, and fever. His medical history is significant for severe chronic obstructive pulmonary disease (COPD) requiring 2 L/min home oxygen, and cor pulmonale. His temperature is 38.6°C (101.5°F), pulse is 112/min, blood pressure is 90/60 mm Hg, and respirations are 26/min. Physical examination reveals a somnolent patient utilizing accessory muscles of respiration. Auscultation of the lungs reveals diffuse expiratory wheezing and coarse crackles in the right lower lobe.

Laboratory results from a routine outpatient clinic visit one month ago, when the patient was at his clinical baseline, showed:

Test	Result
Arterial blood gas (baseline, room air)
$pH$	$7.38$
$pCO_2$	$56 \text{ mm Hg}$
$pO_2$	$62 \text{ mm Hg}$
$HCO_3^-$	$32 \text{ mEq/L}$

Current laboratory results obtained upon arrival are as follows:

Test	Result
Serum chemistry
Sodium	$140 \text{ mEq/L}$
Potassium	$4.2 \text{ mEq/L}$
Chloride	$96 \text{ mEq/L}$
Bicarbonate	$14 \text{ mEq/L}$
Blood urea nitrogen	$30 \text{ mg/dL}$
Creatinine	$1.5 \text{ mg/dL}$
L-lactate	$5.5 \text{ mmol/L}$
Arterial blood gas (room air)
$pH$	$7.15$
$pCO_2$	$42 \text{ mm Hg}$
$pO_2$	$48 \text{ mm Hg}$

Which of the following best characterizes this patient's current acid-base status?

High anion gap metabolic acidosis and respiratory acidosis

High anion gap metabolic acidosis, respiratory acidosis, and metabolic alkalosis

High anion gap metabolic acidosis and respiratory alkalosis

High anion gap metabolic acidosis with appropriate respiratory compensation

Normal anion gap metabolic acidosis and respiratory acidosis

Show answer & explanation

Answer: High anion gap metabolic acidosis and respiratory acidosis

Answer

High anion gap metabolic acidosis and respiratory acidosis

This patient has a primary high anion gap metabolic acidosis (HAGMA) due to sepsis-induced lactic acidosis, evidenced by an anion gap of 30. Using Winter's formula (Expected pCO2 = 1.5 × 14 + 8 = 29 ± 2 mm Hg), the actual pCO2 of 42 mm Hg is significantly higher than expected, indicating a concurrent respiratory acidosis. Finally, calculating the delta-adjusted bicarbonate yields 32 mEq/L (Measured HCO3 [14] + Delta AG [18]). Because 32 mEq/L perfectly matches the patient's documented chronic baseline bicarbonate (a compensatory metabolic alkalosis from chronic COPD), there is no newly superimposed metabolic disorder.

Step-by-Step Solution

1

Calculate the serum Anion Gap (AG).

AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (96 + 14) = 30

.

An

AG > 12

indicates the presence of a primary high anion gap metabolic acidosis (HAGMA), which in this case is driven by severe lactic acidosis (

5.5 \text{ mmol/L}

) secondary to sepsis.

2

Calculate expected respiratory compensation using Winter's formula.

Expected

pCO_2 = 1.5 \times HCO_3^- + 8 \pm 2 = 1.5 \times (14) + 8 = 29 \pm 2 \text{ mm Hg}

.

This establishes the target

pCO_2

for a pure metabolic acidosis. Because the patient's actual

pCO_2

is

42 \text{ mm Hg}

(which is

> 31 \text{ mm Hg}

), he has a concurrent, superimposed respiratory acidosis due to COPD preventing adequate hyperventilation.

3

Calculate the

\Delta

Gap to evaluate for a third, hidden metabolic disorder.

\Delta AG = \text{Actual } AG - \text{Normal } AG = 30 - 12 = 18

. The

\Delta

-adjusted

HCO_3^-

= Measured

HCO_3^- + \Delta AG = 14 + 18 = 32 \text{ mEq/L}

.

The

\Delta

-adjusted

HCO_3^-

reveals what the patient's bicarbonate would be if the high anion gap acids were removed.

4

Compare the

\Delta

-adjusted

HCO_3^-

to the patient's proper baseline.

The calculated

\Delta

-adjusted

HCO_3^-

(

32 \text{ mEq/L}

) is identical to the patient's known chronic baseline

HCO_3^-

(

32 \text{ mEq/L}

).

Because the adjusted value matches his chronic baseline, there is NO additional metabolic acid-base disorder. Comparing it to the population normal (

24 \text{ mEq/L}

) would falsely suggest a metabolic alkalosis.

Key Concept

When analyzing mixed acid-base disorders in a patient with a known chronic acid-base abnormality (e.g., chronic respiratory acidosis), the delta-adjusted bicarbonate must be compared to the patient's established baseline bicarbonate, not the standard population normal of 24 mEq/L.

Estimated Time:2m 0s

Question 22Question →

A fundamental aspect of diagnosing renal pathology involves interpreting urine microscopy. Match each of the following clinical renal conditions with its characteristic, classic urinalysis sediment finding.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Acute tubular necrosis pairs with muddy brown granular casts. Acute interstitial nephritis pairs with WBC casts and eosinophiluria. Glomerulonephritis pairs with RBC casts and dysmorphic RBCs. End-stage renal disease pairs with broad waxy casts.

Each urinalysis finding is directly linked to the underlying pathophysiology of the specific renal disease, serving as a highly specific diagnostic marker for that condition.

Step-by-Step Solution

1

Analyze the pathophysiology of Acute tubular necrosis (ATN)

Identified as necrosis of tubular cells

Necrotic cells form muddy brown granular casts in the tubular lumen.

2

Analyze the pathophysiology of Acute interstitial nephritis (AIN)

Identified as an inflammatory/allergic reaction

Inflammation introduces white blood cells and eosinophils into the urine, forming WBC casts.

3

Analyze the pathophysiology of Glomerulonephritis

Identified as glomerular barrier breakdown

Disrupted glomeruli allow RBCs to leak through, becoming dysmorphic and trapping in casts.

4

Analyze the pathophysiology of End-stage renal disease (ESRD)

Identified as chronic structural changes with dilated tubules

Stasis in enlarged tubules allows for the formation of large, broad waxy casts.

Key Concept

Urine microscopy provides crucial diagnostic clues for localizing the exact site of renal injury (e.g., glomerulus, tubules, or interstitium).

Estimated Time:45s

Question 23Question →

An 18-year-old female university student is evaluated in the emergency department after her roommate found her to be acutely disoriented. Over the last 12 hours, the patient has experienced continuous nausea, episodes of vomiting, and a persistent buzzing sensation in her ears. The roommate suspects she ingested a large quantity of an over-the-counter medication following a personal dispute. Her temperature is $37.8^\circ C$ ( $100^\circ F$ ), blood pressure is $114/72$ mm Hg, pulse is $118$ /min, and respirations are $26$ /min. Physical examination shows a diaphoretic, agitated young woman with deep, rapid breathing.

Laboratory results are as follows:

Laboratory Test	Result
Serum sodium	$140$ mEq/L
Serum potassium	$3.8$ mEq/L
Serum chloride	$104$ mEq/L
Serum bicarbonate	$12$ mEq/L
Blood urea nitrogen	$18$ mg/dL
Serum creatinine	$1.0$ mg/dL
Arterial pH	$7.42$
Arterial $PaO_2$	$98$ mm Hg
Arterial $PaCO_2$	$19$ mm Hg

Which of the following best describes this patient's underlying acid-base disturbance?

Mixed high anion gap metabolic acidosis and primary respiratory alkalosis

High anion gap metabolic acidosis with appropriate respiratory compensation

Primary respiratory alkalosis with appropriate metabolic compensation

Mixed normal anion gap metabolic acidosis and primary respiratory alkalosis

Mixed high anion gap metabolic acidosis and metabolic alkalosis

Show answer & explanation

Answer: Mixed high anion gap metabolic acidosis and primary respiratory alkalosis

Answer

Mixed high anion gap metabolic acidosis and primary respiratory alkalosis

The correct answer identifies a mixed high anion gap metabolic acidosis and a primary respiratory alkalosis. This patient's clinical presentation (tinnitus, tachypnea, nausea, vomiting, agitation) is highly suspicious for salicylate toxicity (aspirin overdose). The acid-base workup reveals a severely low bicarbonate (

12

mEq/L) with an elevated anion gap of

24

mEq/L, confirming a high anion gap metabolic acidosis. Utilizing Winter's formula (Expected

PaCO_2 = (1.5 \times 12) + 8 = 26 \pm 2

), the expected

PaCO_2

for respiratory compensation is

24-28

mm Hg. The actual

PaCO_2

is

19

mm Hg, which is lower than expected, confirming an independent, concurrent primary respiratory alkalosis.

Step-by-Step Solution

1

Evaluate the patient's arterial blood gas to identify the predominant pH direction and primary disturbances.

The pH is

7.42

, which is normal but slightly alkalemic. The

PaCO_2

is severely low (

19

mm Hg), indicating respiratory alkalosis, and the

HCO_3^-

is significantly low (

12

mEq/L), indicating metabolic acidosis.

Understanding the individual components helps recognize that two opposing primary processes are occurring simultaneously, which is characteristic of salicylate toxicity.

2

Calculate the serum anion gap to classify the metabolic acidosis.

Anion Gap

= Na^+ - (Cl^- + HCO_3^-) = 140 - (104 + 12) = 24

mEq/L. This reveals a high anion gap metabolic acidosis (HAGMA).

A high anion gap confirms the presence of unmeasured anions, which in this case are organic acids (lactic acid and ketoacids) secondary to salicylate-induced uncoupling of oxidative phosphorylation.

3

Apply Winter's formula to calculate the expected respiratory compensation for the degree of metabolic acidosis.

Expected

PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2

. Calculation:

(1.5 \times 12) + 8 = 26 \pm 2

mm Hg. The expected range is

24-28

mm Hg.

Winter's formula determines if the patient's respiratory drive is appropriately compensating for the metabolic acidosis.

4

Compare the actual

PaCO_2

to the expected

PaCO_2

range.

The actual

PaCO_2

(

19

mm Hg) is significantly lower than the expected compensatory range (

24-28

mm Hg).

A

PaCO_2

lower than expected indicates excessive hyperventilation, diagnosing an independent, concurrent primary respiratory alkalosis due to direct salicylate stimulation of the medullary respiratory center.

Key Concept

Patients with acute salicylate toxicity classically present with a mixed acid-base disorder: a primary respiratory alkalosis (due to direct medullary respiratory center stimulation) and a concurrent high anion gap metabolic acidosis (due to uncoupling of oxidative phosphorylation and accumulation of organic acids).

Question 24Question →

Paramedics deliver a 68-year-old nursing home resident to the acute care bay after staff found him profoundly lethargic amid multiple episodes of bilious emesis over the preceding 48 hours. Medical history is significant for heart failure with preserved ejection fraction, severe osteoarthritis, and recently diagnosed type 2 diabetes mellitus. On initial assessment, he is obtunded but protects his airway.

Vitals:

Parameter	Value
Temperature	36.8°C (98.2°F)
Heart rate	118/min
Blood pressure	88/54 mm Hg
Respiratory rate	10/min
SpO2	93% (room air)

Laboratory data:

Test	Result
Sodium	142 mEq/L
Potassium	3.1 mEq/L
Chloride	90 mEq/L
Bicarbonate	16 mEq/L
Blood urea nitrogen	58 mg/dL
Creatinine	2.7 mg/dL
Glucose	512 mg/dL

Arterial blood gas (room air):

Parameter	Value
pH	7.23
PaO2	74 mm Hg
PaCO2	40 mm Hg

Assuming a normal reference anion gap of 12 mEq/L and a normal reference serum bicarbonate of 24 mEq/L, what is this patient's delta ratio ( $\Delta AG / \Delta HCO_3^-$ )?

Show answer & explanation

Answer: 3

Answer

The patient's calculated delta ratio is 3.0.

This patient presents with a severe triple acid-base disorder: high anion gap metabolic acidosis (likely diabetic ketoacidosis or lactic acidosis from hypoperfusion), metabolic alkalosis (from severe vomiting), and respiratory acidosis (from obtundation causing hypoventilation). The delta ratio compares the rise in the anion gap (

\Delta AG

) to the fall in bicarbonate (

\Delta HCO_3^-

). For every 1 mEq/L increase in unmeasured acids, the bicarbonate should theoretically fall by 1 mEq/L, yielding a ratio of 1.0 (or up to 2.0 in lactic acidosis due to varying intracellular buffering efficiency). A ratio

> 2.0

indicates that the bicarbonate has not fallen as much as expected given the massive acid load. In this patient, the massive acid load (

\Delta AG = 24

) only dropped the bicarbonate by 8 mEq/L. This proves the existence of a concurrent metabolic alkalosis (from the loss of gastric

HCl

) generating additional bicarbonate that masks the severity of the acidosis. Additionally, Winter's formula predicts a PaCO2 of

1.5(16) + 8 = 32 \pm 2

mm Hg; the measured PaCO2 of 40 mm Hg confirms a concurrent respiratory acidosis.

Step-by-Step Solution

1

Calculate the serum anion gap using the formula

AG = Na^+ - (Cl^- + HCO_3^-)

.

142 - (90 + 16) = 142 - 106 = 36

mEq/L.

The first step in evaluating any metabolic acidosis is determining if unmeasured anions are present.

2

Calculate the change in anion gap (

\Delta AG

).

36 - 12 = 24

mEq/L.

This determines the exact magnitude of excess unmeasured acid that has accumulated in the blood above the normal baseline.

3

Calculate the change in bicarbonate (

\Delta HCO_3^-

).

24 - 16 = 8

mEq/L.

This determines how much the serum bicarbonate has dropped from its baseline normal value due to buffering the acid load.

4

Calculate the delta ratio by dividing

\Delta AG

by

\Delta HCO_3^-

.

24 / 8 = 3.0

.

Evaluating the ratio between the rise in unmeasured acids and the fall in bicarbonate helps identify concurrent secondary metabolic acid-base disorders.

Key Concept

In patients with a high anion gap metabolic acidosis, the delta ratio (

\Delta AG / \Delta HCO_3^-

) is calculated to uncover hidden concurrent metabolic disorders. A ratio greater than 2.0 indicates a superimposed metabolic alkalosis.

Estimated Time:3m 0s

Question 25Question →

A 32-year-old woman is evaluated in the clinic for progressive generalized muscle weakness and fatigue over the past three weeks. She has a past medical history of two episodes of nephrolithiasis in the last three years but takes no daily medications. She reports no recent history of diarrhea, vomiting, or dietary changes. Her temperature is $37.0°\text{C}$ ( $98.6°\text{F}$ ), blood pressure is $112/72 \text{ mm Hg}$ , pulse is $82/\text{min}$ , and respiratory rate is $18/\text{min}$ . Physical examination reveals decreased deep tendon reflexes globally but is otherwise unremarkable.

Laboratory results are as follows:

Test	Result	Reference Range
Sodium ( $Na^+$ )	$140 \text{ mEq/L}$	$136-145 \text{ mEq/L}$
Potassium ( $K^+$ )	$2.7 \text{ mEq/L}$	$3.5-5.0 \text{ mEq/L}$
Chloride ( $Cl^-$ )	$116 \text{ mEq/L}$	$95-105 \text{ mEq/L}$
Bicarbonate ( $HCO_3^-$ )	$14 \text{ mEq/L}$	$22-28 \text{ mEq/L}$
Blood urea nitrogen	$12 \text{ mg/dL}$	$7-18 \text{ mg/dL}$
Creatinine	$0.9 \text{ mg/dL}$	$0.6-1.2 \text{ mg/dL}$

Arterial blood gas (room air):

Test	Result	Reference Range
pH	$7.27$	$7.35-7.45$
$PaCO_2$	$29 \text{ mm Hg}$	$35-45 \text{ mm Hg}$
$PaO_2$	$96 \text{ mm Hg}$	$75-105 \text{ mm Hg}$

Urinalysis reveals a pH of 6.5, specific gravity of 1.015, and is negative for protein, blood, and glucose.

Based on the clinical presentation and laboratory findings, which of the following best describes this patient's underlying condition and acid-base status?

Normal anion gap metabolic acidosis with appropriate respiratory compensation due to a distal tubular defect

High anion gap metabolic acidosis with appropriate respiratory compensation due to unmeasured endogenous acids

Mixed normal anion gap metabolic acidosis and primary respiratory alkalosis due to a proximal tubular defect

Mixed normal anion gap metabolic acidosis and primary respiratory acidosis due to gastrointestinal bicarbonate loss

Normal anion gap metabolic acidosis with inadequate respiratory compensation due to reduced adrenal aldosterone secretion

Show answer & explanation

Answer: Normal anion gap metabolic acidosis with appropriate respiratory compensation due to a distal tubular defect

Answer

Normal anion gap metabolic acidosis with appropriate respiratory compensation due to a distal tubular defect

The patient's laboratory results demonstrate a normal anion gap metabolic acidosis (NAGMA) with appropriate respiratory compensation, secondary to Type 1 (distal) renal tubular acidosis (RTA). A step-wise evaluation reveals acidemia (pH 7.27) with a low bicarbonate (

14 \text{ mEq/L}

), signifying metabolic acidosis. The anion gap is normal at

10 \text{ mEq/L}

. Using Winter's formula, the expected

PaCO_2

is calculated as

1.5 \times 14 + 8 = 29 \pm 2 \text{ mm Hg}

. Because the actual

PaCO_2

is exactly

29 \text{ mm Hg}

, the respiratory compensation is fully appropriate, meaning there is no mixed acid-base disorder. The underlying etiology is a distal tubular defect (Type 1 RTA), which is clinically characterized by severe hypokalemia, the inability to acidify urine below a pH of 5.5 (patient's urine pH is 6.5), and a high risk of calcium phosphate kidney stones forming in the alkaline urine.

Step-by-Step Solution

1

Identify the primary acid-base disturbance by evaluating the arterial pH and serum bicarbonate.

The low pH (

7.27

) and low serum bicarbonate (

14 \text{ mEq/L}

) confirm a primary metabolic acidosis.

Establishing the primary disturbance directs the subsequent steps of the diagnostic algorithm.

2

Calculate the serum anion gap (AG) to classify the etiology of the metabolic acidosis.

The calculated AG is

[Na^+] - ([Cl^-] + [HCO_3^-]) = 140 - (116 + 14) = 10 \text{ mEq/L}

, which falls within the normal range (

10-14 \text{ mEq/L}

).

Differentiating between a normal and high anion gap significantly narrows the differential diagnosis.

3

Apply Winter's formula to determine if the respiratory compensation is appropriate or if a mixed disorder is present.

Expected

PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = 1.5 \times 14 + 8 \pm 2 = 29 \pm 2 \text{ mm Hg}

. The patient's actual

PaCO_2

is

29 \text{ mm Hg}

, confirming perfect respiratory compensation with no secondary respiratory disorder.

Failure to accurately calculate the expected compensatory response frequently leads to the misdiagnosis of a mixed acid-base disorder.

4

Determine the specific cause of the normal anion gap metabolic acidosis based on clinical history, serum potassium, and urine pH.

The presence of severe hypokalemia (

2.7 \text{ mEq/L}

), an inappropriately alkaline urine (pH

6.5

) despite systemic acidemia, and a history of nephrolithiasis strongly point to Type 1 (distal) renal tubular acidosis.

These findings reflect the specific pathophysiological defect of impaired distal tubular hydrogen ion secretion, which inherently prevents proper urine acidification and predisposes to calcium phosphate stone formation.

Key Concept

Type 1 (distal) renal tubular acidosis presents as a normal anion gap metabolic acidosis with hypokalemia, an inappropriately alkaline urine (pH > 5.5) despite systemic acidemia, and a strong predisposition to calcium phosphate nephrolithiasis.

Estimated Time:1m 30s

Question 26Question →

Match each clinical presentation of an acid-base disturbance to its most corresponding arterial blood gas (ABG) profile. Assume normal reference ranges for ABG are pH $7.35-7.45$ , $PaCO_2$ $35-45$ mm Hg, and $HCO_3^-$ $22-26$ mEq/L.

Click a left item, then click its matching right item

Show answer & explanation

Answer

The clinical scenarios perfectly map to their primary acid-base derangements: DKA to metabolic acidosis (pH

7.24

), vomiting to metabolic alkalosis (pH

7.51

), COPD exacerbation to respiratory acidosis (pH

7.29

), and hyperventilation from anxiety to respiratory alkalosis (pH

7.58

).

Each clinical scenario corresponds to a classic acid-base disturbance. Diabetic ketoacidosis generates ketones, causing an anion gap metabolic acidosis. Severe vomiting causes loss of hydrogen and chloride ions, producing a hypochloremic metabolic alkalosis. COPD exacerbations lead to hypoventilation and

CO_2

retention (respiratory acidosis). Acute anxiety provokes hyperventilation, blowing off

CO_2

and resulting in respiratory alkalosis. Matching the primary directional change in pH,

PaCO_2

, and

HCO_3^-

while verifying expected compensation formulas confirms the correct associations.

Step-by-Step Solution

1

Identify the primary acid-base disturbance based on the clinical history.

DKA causes metabolic acidosis; vomiting causes metabolic alkalosis; COPD exacerbation causes respiratory acidosis; severe anxiety/hyperventilation causes respiratory alkalosis.

Determines the expected directional changes in pH,

PaCO_2

, and

HCO_3^-

before looking at the specific numeric options.

2

Analyze the ABG profiles to find the primary disorder by matching pH with the corresponding primary derangement.

pH

7.24

with low

HCO_3^-

is metabolic acidosis. pH

7.51

with high

HCO_3^-

is metabolic alkalosis. pH

7.29

with high

PaCO_2

is respiratory acidosis. pH

7.58

with low

PaCO_2

is respiratory alkalosis.

Links the clinical condition directly to the correct ABG pattern.

3

Verify appropriate physiological compensation for each disorder using standard formulas.

Winter's formula for the DKA ABG yields an expected

PaCO_2

of

23 \pm 2

mm Hg. Alkalosis compensation yields a

PaCO_2

around

48

mm Hg. Acute respiratory alkalosis drops

HCO_3^-

to exactly

20

mEq/L.

Confirms the exact ABG profile matches the pure nature of the disorder and validates the initial matching.

Key Concept

Diagnosis of acid-base disorders requires correlating the clinical history with arterial blood gas primary derangements (metabolic vs. respiratory) and calculating expected physiological compensations.

Alternative Method

Instead of executing full compensation calculations for every option, simply identifying the primary disturbance (e.g., pH <

7.35

and low

HCO_3^-

strictly defines a metabolic acidosis) is often sufficient to uniquely map each clinical scenario to its corresponding ABG profile in a single-best-match format.

Estimated Time:2m 0s

Question 27Question →

Review the clinical vignettes below, which present various mechanisms of renal and systemic acid-base derangements. Which classic arterial blood gas and serum chemistry profile is most strongly associated with each patient's presentation?

Click a left item, then click its matching right item

Show answer & explanation

Answer

The correct matches pair the specific mechanism of acid-base disturbance with its hallmark laboratory profile. The patient with vomiting has a hypokalemic metabolic alkalosis. The patient with an ileostomy has a normal anion gap acidosis. The patient in diabetic ketoacidosis has an elevated anion gap acidosis with hyperkalemia. The COPD patient has an acute-on-chronic respiratory acidosis. The patient with aspirin toxicity has a dual presentation of an elevated anion gap acidosis and independent respiratory-driven alkalemia.

Matching questions require correctly identifying the physiological consequences of each clinical event. Vomiting uniquely causes a hypokalemic metabolic alkalosis due to direct acid loss. Lower gastrointestinal losses (ileostomy) cause a normal anion gap acidosis due to direct bicarbonate wasting. Ketoacidosis produces a severe elevated anion gap acidosis accompanied by hyperkalemia due to extracellular shifts. Chronic respiratory failure acute exacerbations cause acute-on-chronic respiratory acidosis. Salicylate toxicity uniquely produces a dual-primary disorder: a respiratory-driven alkalemia from central stimulation and an elevated anion gap acidosis from cellular toxicity.

Step-by-Step Solution

1

Calculate the serum anion gap (

AG = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-)

) for each provided laboratory profile to categorize the metabolic disturbances.

The third and fifth profiles reveal elevated anion gaps (

26 \text{ mEq/L}

and

25 \text{ mEq/L}

respectively). The second profile reveals a normal anion gap (

11 \text{ mEq/L}

).

Identifying the anion gap is the critical first step in determining the etiology of any metabolic acidosis.

2

Evaluate the patient with severe vomiting and volume depletion.

Gastric acid loss generates a severe metabolic alkalosis with compensatory hypoventilation. This perfectly corresponds to the first profile featuring a

\text{pH}

of

7.52

, elevated bicarbonate, and profound hypochloremia.

Vomiting classically leads to a hypokalemic, hypochloremic metabolic alkalosis due to direct acid loss and secondary hyperaldosteronism from volume contraction.

3

Analyze the patient with a high-output ileostomy.

This patient loses massive amounts of bicarbonate from the lower GI tract, producing a hyperchloremic, normal anion gap acidosis, matching the second profile.

Gastrointestinal fluid below the stomach is rich in bicarbonate; its excessive loss forces the kidneys to retain chloride to maintain electroneutrality.

4

Assess the patient with insulin deprivation (diabetic ketoacidosis).

DKA produces an elevated anion gap acidosis. The profound acidemia (

\text{pH}

7.22

), elevated gap (

26 \text{ mEq/L}

), and hyperkalemia (

6.1 \text{ mEq/L}

) despite total body potassium depletion match the third profile.

Insulin deficiency leads to unchecked lipolysis and ketone body production, while extracellular potassium shifts occur due to lack of insulin and profound acidemia.

5

Examine the patient with an acute COPD exacerbation.

This presentation is characterized by acute-on-chronic respiratory acidosis. The fourth profile demonstrates a severely elevated

\text{pCO}_2

(

70 \text{ mmHg}

) and acidemia, with chronically retained bicarbonate (

31 \text{ mEq/L}

).

Chronic hypercapnia leads to renal bicarbonate retention over days to weeks. An acute exacerbation spikes the

\text{pCO}_2

further, dropping the pH precipitously.

6

Determine the acid-base consequence of aspirin toxicity in the final patient.

Salicylates directly stimulate the respiratory center (driving

\text{pCO}_2

down) while uncoupling oxidative phosphorylation (elevating the anion gap). This dual pathology matches the fifth profile, which shows a normal-to-alkalemic pH (

7.42

) despite a very low bicarbonate (

13 \text{ mEq/L}

) and an elevated anion gap.

The presence of two competing, simultaneous primary disorders (respiratory alkalemia and metabolic acidemia) is the hallmark of adult salicylate poisoning.

Key Concept

Pathophysiological correlation of complex clinical presentations with their expected arterial blood gas and serum electrolyte profiles.

Question 28Question →

An emergency department evaluation is requested for a 58-year-old male presenting with progressively worsening shortness of breath and a cough producing frothy sputum over the past 48 hours. His medical history is significant for a 30-pack-year smoking habit and type 2 diabetes mellitus complicated by diabetic nephropathy. Upon arrival, he is in moderate respiratory distress.

Vital signs reveal a blood pressure of $185/110 \text{ mm Hg}$ , pulse of $112/\text{min}$ , and respiratory rate of $28/\text{min}$ . Oxygen saturation is 88% on room air. Pulmonary auscultation reveals bilateral crackles extending to the mid-lung fields along with diffuse expiratory wheezing. Examination of the lower extremities shows 2+ bilateral pitting edema.

Laboratory evaluation reveals the following:
Serum sodium: $138 \text{ mEq/L}$
Serum potassium: $5.9 \text{ mEq/L}$
Serum chloride: $102 \text{ mEq/L}$
Serum bicarbonate: $12 \text{ mEq/L}$
Blood urea nitrogen: $92 \text{ mg/dL}$
Serum creatinine: $7.4 \text{ mg/dL}$

Arterial blood gas (room air):
pH: $7.22$
$p\text{CO}_2$ : $36 \text{ mm Hg}$
$p\text{O}_2$ : $58 \text{ mm Hg}$

Based on the clinical and laboratory findings, which of the following best describes this patient's current acid-base disorder and the primary cause of his respiratory distress?

Metabolic acidosis with an elevated anion gap and a concurrent respiratory acidosis; distress is primarily due to acute volume overload

Metabolic acidosis with an elevated anion gap and appropriate respiratory compensation; distress is primarily due to acute volume overload

Metabolic acidosis with an elevated anion gap and a concurrent respiratory acidosis; distress is primarily due to a chronic obstructive pulmonary disease exacerbation

Metabolic acidosis with an elevated anion gap and appropriate respiratory compensation; distress is primarily due to a chronic obstructive pulmonary disease exacerbation

Metabolic acidosis with an elevated anion gap and a concurrent respiratory alkalosis; distress is primarily due to acute volume overload

Show answer & explanation

Answer: Metabolic acidosis with an elevated anion gap and a concurrent respiratory acidosis; distress is primarily due to acute volume overload

Answer

Metabolic acidosis with an elevated anion gap and a concurrent respiratory acidosis; distress is primarily due to acute volume overload

The patient has a metabolic acidosis with an elevated anion gap of 24, driven by his acute-on-chronic renal failure (uremia). Using Winter's formula (

1.5 \times [\text{HCO}_3^-] + 8 \pm 2

), the expected

p\text{CO}_2

is

24-28 \text{ mm Hg}

. The measured

p\text{CO}_2

of

36 \text{ mm Hg}

is substantially higher than this range, revealing a concurrent respiratory acidosis. The patient's profound hypertension, severe renal failure, crackles, pitting edema, and frothy sputum all indicate acute volume overload leading to pulmonary edema. The fluid accumulation in the lungs causes respiratory fatigue and

\text{CO}_2

retention, alongside 'cardiac asthma' (wheezing due to peribronchial edema).

Step-by-Step Solution

1

Calculate the serum anion gap to determine the primary metabolic disorder.

Anion gap =

138 - (102 + 12) = 24

.

Identifies the presence of a metabolic acidosis with an elevated anion gap, likely secondary to uremia given the markedly elevated BUN and creatinine.

2

Apply Winter's formula to determine the expected

p\text{CO}_2

for appropriate respiratory compensation.

Expected

p\text{CO}_2 = (1.5 \times 12) + 8 \pm 2 = 26 \pm 2 \text{ mm Hg}

.

Evaluates if a secondary acid-base disorder is present alongside the primary metabolic acidosis.

3

Compare the measured

p\text{CO}_2

to the calculated expected

p\text{CO}_2

.

The measured

p\text{CO}_2

of

36 \text{ mm Hg}

is significantly higher than the expected range (

24-28 \text{ mm Hg}

).

A higher-than-expected

p\text{CO}_2

indicates a concurrent respiratory acidosis due to inadequate ventilation.

4

Analyze the physical exam findings to identify the cause of respiratory distress and the respiratory acidosis.

Severe hypertension, frothy sputum, bilateral crackles, pitting edema, and severe renal failure point directly to acute pulmonary edema.

Volume overload leads to pulmonary edema, which impairs gas exchange and causes respiratory fatigue. This explains both the wheezing ('cardiac asthma') and the concurrent respiratory acidosis.

Key Concept

Mixed acid-base disorders and differentiation of acute pulmonary edema from primary obstructive lung disease.

Question 29Question →

A 55-year-old man presents to the emergency department with a 4-day history of profuse watery diarrhea. He reports orthostatic dizziness, generalized weakness, and a significantly reduced urine output over the past 24 hours. He has no significant past medical history and takes no medications. Vital signs show a temperature of $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ ), blood pressure of $88/55\text{ mm Hg}$ , and a heart rate of $122/\text{min}$ . Physical examination reveals dry mucous membranes, decreased skin turgor, and flat jugular veins.

Laboratory studies are as follows:

Serum
Sodium: $140\text{ mEq/L}$
Potassium: $3.2\text{ mEq/L}$
Chloride: $110\text{ mEq/L}$
Bicarbonate: $18\text{ mEq/L}$
Blood urea nitrogen (BUN): $60\text{ mg/dL}$
Creatinine: $2.5\text{ mg/dL}$

Urine
Sodium: $14\text{ mEq/L}$
Creatinine: $50\text{ mg/dL}$

What is this patient's fractional excretion of sodium ( $Fe_{Na}$ ) in percentage?

Show answer & explanation

Answer: 0.5

Answer

0.5

The fractional excretion of sodium (

Fe_{Na}

) measures the percentage of filtered sodium that is eventually excreted in the urine. It is widely used to distinguish between prerenal acute kidney injury (AKI) and acute tubular necrosis (ATN). It is calculated using the formula:

Fe_{Na} = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100

. Plugging in the patient's laboratory values yields

\frac{14 \times 2.5}{140 \times 50} \times 100 = 0.5\%

. In the setting of oliguria, an

Fe_{Na} < 1\%

typically indicates prerenal azotemia, representing an intact renal tubular mechanism trying to conserve sodium and water in response to hypovolemia (consistent with this patient's severe dehydration from diarrhea).

Step-by-Step Solution

1

Identify the values required for the FeNa calculation.

U_{Na} = 14

,

P_{Cr} = 2.5

,

P_{Na} = 140

,

U_{Cr} = 50

These specific variables are needed to use the

Fe_{Na}

equation, which helps differentiate prerenal azotemia from intrinsic renal disease.

2

Apply the FeNa formula.

Fe_{Na} = \frac{14 \times 2.5}{140 \times 50} \times 100

The standard formula requires multiplying the ratio of urine-to-plasma sodium by the ratio of plasma-to-urine creatinine.

3

Calculate the final percentage.

0.5\%

This calculation yields the final percentage, which is less than

1\%

and highly indicative of a prerenal etiology.

Key Concept

Fractional Excretion of Sodium (FeNa) Calculation

Question 30Question →

Match each clinical and laboratory presentation with the most likely underlying nephrological etiology.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Presentation 1 matches Multiple myeloma cast nephropathy. Presentation 2 matches IgA nephropathy. Presentation 3 matches Type 1 (distal) renal tubular acidosis. Presentation 4 matches Infective endocarditis-associated glomerulonephritis. Presentation 5 matches Drug-induced acute interstitial nephritis.

Each clinical presentation is matched based on pathognomonic findings: myeloma cast nephropathy shows a dipstick-SSA protein discrepancy; IgA nephropathy presents with synpharyngitic hematuria and normal complement; Type 1 RTA presents with NAGMA, hypokalemia, and a positive urine anion gap; endocarditis-associated GN presents with hypocomplementemia and RBC casts; and AIN presents with an allergic triad and WBC casts.

Step-by-Step Solution

1

Analyze the urinalysis protein discrepancy and clinical signs in presentation 1.

Trace dipstick but strongly positive SSA test, combined with hypercalcemia and back pain, indicates the presence of monoclonal light chains.

This is a classic pathognomonic finding for multiple myeloma cast nephropathy.

2

Evaluate the timing of hematuria relative to infection and complement levels in presentation 2.

Hematuria is concurrent with a viral infection (synpharyngitic) and C3/C4 levels are normal.

These specific findings distinguish IgA nephropathy from post-infectious glomerulonephritis, which typically occurs weeks later and presents with low C3 levels.

3

Assess the acid-base and electrolyte derangements in presentation 3.

Patient has a normal anion gap metabolic acidosis, profound hypokalemia, alkaline urine (pH

6.5

), and a positive urine anion gap in the setting of Sjögren syndrome.

This points to a failure of distal tubular acid secretion, confirming Type 1 (distal) renal tubular acidosis.

4

Identify the systemic syndrome and correlating renal findings in presentation 4.

Fever, a mechanical valve, petechiae, acute kidney injury, RBC casts, and low complement levels.

This constellation is diagnostic of infective endocarditis complicated by immune-complex mediated glomerulonephritis.

5

Correlate the clinical triad and medication history in presentation 5.

Recent piperacillin-tazobactam use followed by a diffuse rash, acute kidney injury, and WBC casts.

This represents an allergic inflammatory response characteristic of drug-induced acute interstitial nephritis.

Key Concept

Differentiation of nephrological etiologies using classic clinical presentations, timing of symptoms, complement levels, acid-base profiles, and specific urinalysis findings.

Estimated Time:2m 30s

Question 31Question →

A 21-year-old female university student is evaluated in the urgent care clinic after her roommates noticed her acting increasingly confused and breathing rapidly over the past $4\text{ hours}$ . She has a history of major depressive disorder and recently failed an important exam. She complains of a constant "buzzing" sound in her ears. Her temperature is $37.9^\circ\text{C}$ ( $100.2^\circ\text{F}$ ), pulse is $118\text{/min}$ , respirations are $30\text{/min}$ , and blood pressure is $112/68\text{ mm Hg}$ . Physical examination reveals an agitated, tachypneic patient with clear lung fields and no focal neurologic deficits.

Laboratory studies reveal the following:

Laboratory Test	Result
Sodium	$140\text{ mEq/L}$
Potassium	$3.5\text{ mEq/L}$
Chloride	$104\text{ mEq/L}$
Bicarbonate	$13\text{ mEq/L}$
Blood urea nitrogen	$18\text{ mg/dL}$
Creatinine	$0.9\text{ mg/dL}$

Arterial Blood Gas	Result
pH	$7.48$
PaCO $_2$	$18\text{ mm Hg}$
PaO $_2$	$98\text{ mm Hg}$

Which of the following best describes this patient's underlying acid-base derangement?

High anion gap metabolic acidosis and concurrent respiratory acidosis

High anion gap metabolic acidosis with appropriate respiratory compensation

Normal anion gap metabolic acidosis and concurrent respiratory alkalosis

High anion gap metabolic acidosis and concurrent respiratory alkalosis

Primary respiratory alkalosis with appropriate metabolic compensation

Show answer & explanation

Answer: High anion gap metabolic acidosis and concurrent respiratory alkalosis

Answer

High anion gap metabolic acidosis and concurrent respiratory alkalosis

The correct answer identifies that this patient has a mixed acid-base disorder characterized by both a high anion gap metabolic acidosis and a primary respiratory alkalosis, which is the classic presentation of acute salicylate (aspirin) toxicity. The clinical context of a suicide attempt, tinnitus ('buzzing' in ears), fever, and tachypnea strongly suggest this diagnosis. The first step in evaluating this patient's acid-base status is calculating the serum anion gap:

\text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (104 + 13) = 23\text{ mEq/L}

. Since the normal range is

10-14\text{ mEq/L}

, a high anion gap metabolic acidosis (HAGMA) is present. Salicylates cause this by uncoupling oxidative phosphorylation and interfering with the Krebs cycle, leading to the accumulation of organic acids. The second step is to use Winter's formula to determine if the respiratory response is an appropriate physiological compensation or if a concurrent respiratory disorder exists. Expected

\text{PaCO}_2 = 1.5 \times [\text{HCO}_3^-] + 8 \pm 2

. For this patient,

1.5 \times 13 + 8 = 27.5 \pm 2\text{ mm Hg}

(range

25.5 - 29.5\text{ mm Hg}

). The patient's measured

\text{PaCO}_2

is

18\text{ mm Hg}

, which is substantially lower than the expected range. This excessive hyperventilation indicates a concurrent primary respiratory alkalosis, caused by salicylates directly stimulating the medullary respiratory center. Therefore, the patient has a mixed HAGMA and respiratory alkalosis.

Step-by-Step Solution

1

Calculate the serum anion gap to evaluate for a primary metabolic acidosis.

Anion Gap = Na - (Cl + HCO3) = 140 - (104 + 13) = 23 mEq/L.

An anion gap > 14 mEq/L establishes the presence of a High Anion Gap Metabolic Acidosis (HAGMA), a key component of salicylate toxicity.

2

Use Winter's formula to determine the expected respiratory compensation (expected PaCO2) for the calculated metabolic acidosis.

Expected PaCO2 = (1.5 * HCO3) + 8 +/- 2 = (1.5 * 13) + 8 +/- 2 = 27.5 +/- 2 mm Hg (Range: 25.5 to 29.5 mm Hg).

Winter's formula determines if the patient's breathing pattern is an appropriate physiologic response to the acidosis or if a second primary acid-base disorder is present.

3

Compare the patient's measured PaCO2 to the expected PaCO2 range.

The measured PaCO2 is 18 mm Hg, which is significantly lower than the expected minimum of 25.5 mm Hg.

Because the measured PaCO2 is lower than what would be expected from compensatory hyperventilation alone, a concurrent primary respiratory alkalosis must also be present.

Key Concept

Patients with salicylate (aspirin) toxicity classically present with a mixed acid-base disorder: a primary respiratory alkalosis (due to direct medullary respiratory center stimulation) and a primary high anion gap metabolic acidosis (due to uncoupling of oxidative phosphorylation).

Question 32Question →

Match each clinical presentation of a nephrologic or urologic emergency with its most appropriate targeted intervention.

Click a left item, then click its matching right item

Show answer & explanation

Answer

The missed dialysis patient requires calcium gluconate; the lithium toxicity patient requires amiloride; the crush injury patient requires aggressive hydration; and the prostatic hyperplasia patient requires a urethral catheter.

Each clinical scenario requires a specific, targeted intervention based on the underlying pathophysiology of the nephrologic or urologic emergency, ranging from membrane stabilization to mechanical obstruction relief.

Step-by-Step Solution

1

Analyze the presentation of the patient who missed hemodialysis.

The patient has life-threatening hyperkalemia causing critical cardiac toxicity, indicated by the sine-wave pattern.

Recognizing severe ECG changes in hyperkalemia prioritizes immediate myocardial membrane stabilization over definitive potassium removal.

2

Analyze the presentation of the patient on long-term lithium therapy.

The patient has nephrogenic diabetes insipidus, indicated by hypernatremia and failure to concentrate urine after receiving exogenous desmopressin.

Identifying the renal resistance to ADH directs specific pharmacotherapy (amiloride) to block lithium uptake in the distal nephron.

3

Analyze the presentation of the crush injury victim.

The patient has severe rhabdomyolysis, leading to myoglobin-induced acute kidney injury.

Early recognition of rhabdomyolysis prompts immediate volume expansion to prevent ischemic and toxic tubular injury.

4

Analyze the presentation of the elderly man with suprapubic pain.

The patient has acute urinary retention due to benign prostatic hyperplasia.

Identifying postrenal obstruction requires immediate mechanical decompression to relieve symptoms and prevent permanent kidney damage.

Key Concept

Targeted management of nephrologic emergencies relies on identifying the primary pathophysiologic insult, such as membrane instability in hyperkalemia or tubular toxicity in rhabdomyolysis.

Estimated Time:1m 30s

Question 33Question →

A 32-year-old woman is evaluated in the outpatient clinic for progressive fatigue and severe muscle weakness that has worsened over the last month. She reports that her weakness is most pronounced in her thighs and shoulders, making it difficult to climb stairs or comb her hair. She has no chronic medical conditions and does not take any prescription or over-the-counter medications. Her blood pressure is 112/74 mm Hg, pulse is 84/min, and respiratory rate is 16/min. Physical examination reveals normal muscle bulk but 3/5 motor strength in the proximal upper and lower extremities bilaterally. Sensation is intact.

Laboratory analysis reveals the following:

Serum chemistry:
$Na^+$ : 140 mEq/L
$K^+$ : 2.6 mEq/L
$Cl^-$ : 114 mEq/L
$HCO_3^-$ : 14 mEq/L
Blood urea nitrogen: 14 mg/dL
Creatinine: 0.8 mg/dL

Arterial blood gas (room air):
$pH$ : 7.29
$pCO_2$ : 29 mm Hg
$pO_2$ : 96 mm Hg

Urinalysis:
$pH$ : 6.5
Specific gravity: 1.012
Ketones: Negative
Protein: Negative

Urine electrolytes:
$Na^+$ : 45 mEq/L
$K^+$ : 15 mEq/L
$Cl^-$ : 25 mEq/L

Which of the following best describes this patient's acid-base disturbance and underlying etiology?

Normal anion gap metabolic acidosis with adequate respiratory compensation due to impaired distal tubular hydrogen secretion

Normal anion gap metabolic acidosis with concurrent respiratory acidosis due to impaired proximal tubular bicarbonate reabsorption

Normal anion gap metabolic acidosis with concurrent respiratory alkalosis due to gastrointestinal bicarbonate loss

High anion gap metabolic acidosis with adequate respiratory compensation due to accumulation of unmeasured organic acids

Normal anion gap metabolic acidosis with adequate respiratory compensation due to renal resistance to aldosterone

Show answer & explanation

Answer: Normal anion gap metabolic acidosis with adequate respiratory compensation due to impaired distal tubular hydrogen secretion

Answer

The patient has a normal anion gap metabolic acidosis with adequate respiratory compensation caused by Type 1 (distal) renal tubular acidosis, characterized by impaired distal tubular hydrogen secretion.

This patient's serum anion gap is

140 - (114 + 14) = 12 \text{ mEq/L}

, indicating a normal anion gap metabolic acidosis (NAGMA). To assess if this is a simple or mixed disorder, Winter's formula must be applied to predict the expected respiratory compensation: expected

pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2

. For this patient,

(1.5 \times 14) + 8 = 29 \text{ mm Hg}

. Because the actual

pCO_2

is exactly 29 mm Hg, the respiratory compensation is fully adequate, and there is no concurrent respiratory disorder. To determine the etiology of the NAGMA, the urine anion gap (

U_{Na} + U_{K} - U_{Cl}

) is calculated. A positive UAG (

45 + 15 - 25 = +35 \text{ mEq/L}

) indicates impaired renal ammonium excretion, identifying Renal Tubular Acidosis (RTA) rather than gastrointestinal bicarbonate loss (which typically produces a negative UAG). The combination of hypokalemia and an inappropriately basic urine pH (6.5) in the face of systemic acidosis is diagnostic of Type 1 (distal) RTA, which occurs due to impaired distal tubular hydrogen ion secretion.

Step-by-Step Solution

1

Calculate the serum Anion Gap (AG) to classify the primary acid-base disorder.

AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (114 + 14) = 12 \text{ mEq/L}

.

An AG of 12 mEq/L is normal. Combined with the low pH (7.29) and low serum bicarbonate (14 mEq/L), this confirms a Normal Anion Gap Metabolic Acidosis (NAGMA).

2

Calculate the expected respiratory compensation using Winter's formula.

Expected

pCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2 \text{ mm Hg}

.

Because the actual

pCO_2

is exactly 29 mm Hg, the patient has adequate respiratory compensation without a superimposed secondary respiratory disorder.

3

Calculate the Urine Anion Gap (UAG) to distinguish between renal and gastrointestinal etiologies of NAGMA.

UAG = U_{Na} + U_{K} - U_{Cl} = 45 + 15 - 25 = +35 \text{ mEq/L}

.

A positive UAG indicates that the kidneys are failing to properly excrete ammonium (

NH_4^+

). This points to a renal etiology (Renal Tubular Acidosis) rather than gastrointestinal bicarbonate loss (which would present with a negative UAG).

4

Evaluate serum potassium and urine pH to determine the specific subtype of Renal Tubular Acidosis.

The patient has severe hypokalemia (

2.6 \text{ mEq/L}

) and an inappropriately high urine

pH

(

6.5

).

Type 1 (distal) RTA is caused by impaired distal

H^+

secretion, leading to an inability to acidify urine (urine

pH > 5.5

) and resultant potassium wasting. Type 4 RTA would present with hyperkalemia, and Type 2 RTA typically has a urine

pH < 5.5

once systemic acidosis is established.

Key Concept

The evaluation of a normal anion gap metabolic acidosis requires calculating the expected respiratory compensation (Winter's formula) to rule out mixed disorders, followed by the urine anion gap (UAG) to differentiate renal tubular acidosis (positive UAG) from gastrointestinal losses (negative UAG).

Question 34Question →

Match the following arterial blood gas and serum electrolyte profiles with their most likely underlying clinical etiology.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Profile 1 corresponds to laxative consumption (normal gap acidosis), Profile 2 to salicylate toxicity (mixed disorder), Profile 3 to insulin deficiency (high gap acidosis), Profile 4 to obstructive airway disease (chronic respiratory acidosis), and Profile 5 to prolonged emesis (metabolic alkalosis).

Each profile uniquely identifies a distinct acid-base disturbance. Applying the principles of anion gap calculation and expected respiratory compensation separates the varying etiologies accurately.

Step-by-Step Solution

1

Calculate the anion gap for every profile using the formula

\text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-)

.

Profile 1:

11

, Profile 2:

23

, Profile 3:

27

, Profile 4:

10

, Profile 5:

10

.

Determining the anion gap is an essential first step in evaluating any acid-base disorder, revealing hidden metabolic derangements even when the pH is alkalemic.

2

Determine the primary acid-base disturbance by examining the pH in conjunction with the changes in PaCO2 and HCO3-.

Profiles 1 and 3 are primary metabolic acidoses. Profile 2 is a mixed primary respiratory alkalosis and primary metabolic acidosis. Profile 4 is a primary respiratory acidosis. Profile 5 is a primary metabolic alkalosis.

Identifying the primary physiological disturbance dictates which secondary compensatory formulas must be applied.

3

Apply Winter's formula (

1.5 \times \text{HCO}_3^- + 8 \pm 2

) to the patients with metabolic acidosis.

For Profile 3, the predicted

\text{PaCO}_2

is

24.5\text{ mmHg}

, matching the measured

25\text{ mmHg}

(simple compensated high gap acidosis). For Profile 2, the predicted

\text{PaCO}_2

is

29\text{ mmHg}

, but the measured is

20\text{ mmHg}

, confirming an additional primary respiratory alkalosis.

This mathematical step definitively distinguishes simple, appropriately compensated acid-base disorders from complex mixed disorders.

4

Correlate the mathematical acid-base diagnoses with the provided pathophysiological states.

Normal gap acidosis matches intestinal bicarbonate loss. High gap acidosis matches ketone production. The mixed respiratory alkalosis and high gap acidosis matches acetylsalicylic acid toxicity. Chronic compensated respiratory acidosis matches COPD. Metabolic alkalosis matches gastric acid loss.

Final confirmation requires aligning the objective laboratory values with the clinical mechanism of disease.

Key Concept

Diagnosis of complex and mixed acid-base disorders using the anion gap, Winter's formula, and compensation rules.

Question 35Question →

Evaluate the following clinical profiles of patients presenting with hypokalemia and acid-base disturbances. Match each patient's laboratory and clinical presentation with the most likely underlying pathophysiologic mechanism.

Click a left item, then click its matching right item

Show answer & explanation

Answer

The hypertensive patient with high urine chloride has autonomous adrenal hypersecretion; the normotensive patient with high urine chloride has a tubular ion channel mutation; the normotensive patient with low urine chloride has surreptitious gastric fluid loss; and the patient with non-anion gap acidosis and positive urine anion gap has a distal tubular acidification defect.

Each clinical scenario corresponds to a distinct pathophysiologic node in the acid-base diagnostic algorithms. Saline-responsive alkalosis (low urine chloride) stems from gastric loss; saline-resistant alkalosis (high urine chloride) splits by blood pressure into mineralocorticoid excess (hypertension) or tubular defects (normotension). A non-anion gap acidosis with a positive urine anion gap points to an acidification defect (Type 1 RTA).

Step-by-Step Solution

1

Evaluate the pH and bicarbonate to determine the primary acid-base disorder.

Three patients have metabolic alkalosis (elevated bicarbonate), while one has a non-anion gap metabolic acidosis (decreased bicarbonate).

Categorizing the primary acid-base disorder dictates the subsequent diagnostic algorithm.

2

For the patients with metabolic alkalosis, analyze the spot urine chloride concentration.

One patient has low urine chloride (< 20 mEq/L), indicating a saline-responsive etiology (gastric loss). Two patients have high urine chloride (> 20 mEq/L), indicating saline-resistant etiologies.

Urine chloride differentiates volume-depleted states (where the kidney avidly reabsorbs chloride) from states of renal chloride wasting or volume expansion.

3

For the patients with high urine chloride metabolic alkalosis, evaluate the blood pressure.

The hypertensive patient has mineralocorticoid excess, while the normotensive patient has a tubular defect.

Mineralocorticoid excess causes sodium retention and hypertension, whereas tubular defects (like Gitelman syndrome) cause salt wasting and normotension/hypotension.

4

For the patient with non-anion gap metabolic acidosis, interpret the urine anion gap.

A positive urine anion gap indicates a renal cause of acidosis.

The positive gap signifies a failure of the kidneys to excrete ammonium (NH4+), which is characteristic of renal tubular acidosis (RTA).

Key Concept

The diagnostic algorithm for metabolic alkalosis relies heavily on spot urine chloride and blood pressure, while the urine anion gap helps diagnose renal tubular acidosis.

Question 36Question →

A 72-year-old man with a history of severe chronic obstructive pulmonary disease (COPD) and heart failure with preserved ejection fraction is brought to the emergency department with worsening shortness of breath and lower extremity edema over the past week. Three days ago, his outpatient physician doubled his daily dose of oral furosemide. On examination, the patient is alert but in mild respiratory distress. Vital signs are: temperature $37.1^\circ\text{C}$ , blood pressure $110/70 \text{ mmHg}$ , heart rate $95/\text{min}$ , and respiratory rate $22/\text{min}$ . Pulse oximetry is $89\%$ on room air.

Initial laboratory studies on room air show:

Test	Value
Arterial $pH$	$7.39$
Arterial $PaCO_2$	$60 \text{ mm Hg}$
Arterial $PaO_2$	$58 \text{ mm Hg}$
Serum $Na^+$	$138 \text{ mEq/L}$
Serum $K^+$	$3.1 \text{ mEq/L}$
Serum $Cl^-$	$92 \text{ mEq/L}$
Serum $HCO_3^-$	$36 \text{ mEq/L}$
Blood urea nitrogen	$38 \text{ mg/dL}$
Serum creatinine	$1.5 \text{ mg/dL}$

Which of the following is the most accurate assessment of this patient's current acid-base status?

Chronic respiratory acidosis and superimposed metabolic alkalosis

Fully compensated chronic respiratory acidosis

Primary metabolic alkalosis with compensatory respiratory acidosis

Acute respiratory acidosis and superimposed metabolic alkalosis

Chronic respiratory acidosis and superimposed metabolic acidosis

Show answer & explanation

Answer: Chronic respiratory acidosis and superimposed metabolic alkalosis

Answer

The patient has a primary chronic respiratory acidosis (due to underlying COPD) with a superimposed metabolic alkalosis (driven by aggressive furosemide therapy leading to contraction alkalosis and hypokalemia).

The correct interpretation involves identifying the primary disturbance and checking compensation. A pH of 7.39 and PaCO2 of 60 mm Hg denotes primary respiratory acidosis. Because the patient has severe COPD, this is chronic. The renal compensation for chronic respiratory acidosis is an increase in HCO3- of 3.5 mEq/L for every 10 mm Hg rise in PaCO2. Here, PaCO2 is 20 above normal (60 - 40). The expected HCO3- = 24 + (2 * 3.5) = 31 mEq/L. The patient's actual HCO3- is 36 mEq/L. Since actual HCO3- > expected HCO3-, there is a superimposed metabolic alkalosis, clinically explained by the recent escalation of loop diuretic therapy causing volume contraction and hypokalemia.

Step-by-Step Solution

1

Determine the primary acid-base disturbance by looking at the pH and PaCO2.

The

pH

is

7.39

(acidemic side of

7.40

), and

PaCO_2

is high at

60 \text{ mm Hg}

. This combination defines a primary respiratory acidosis.

The direction of the pH variation from the absolute normal (

7.40

) indicates the primary disorder. High

PaCO_2

drives the pH down.

2

Determine whether the respiratory acidosis is acute or chronic based on clinical history and standard compensation formulas.

The patient has severe COPD, suggesting chronicity. In chronic respiratory acidosis,

HCO_3^-

increases by

3

to

4 \text{ mEq/L}

for every

10 \text{ mm Hg}

increase in

PaCO_2

above

40

.

Renal compensation for respiratory acidosis takes 3-5 days to fully manifest. Chronic CO2 retainers rely on this mechanism.

3

Calculate the expected compensatory

HCO_3^-

level.

PaCO_2

has risen from

40

to

60 \text{ mm Hg}

(an increase of

20

). The expected increase in

HCO_3^-

is

2 \times 3.5 = 7 \text{ mEq/L}

. Expected

HCO_3^-

is

24 + 7 = 31 \text{ mEq/L}

.

Calculating expected compensation is necessary to unmask any hidden secondary or tertiary acid-base disorders.

4

Compare actual

HCO_3^-

to expected

HCO_3^-

.

The actual

HCO_3^-

is

36 \text{ mEq/L}

, which is significantly higher than the expected

31 \text{ mEq/L}

.

A higher-than-expected bicarbonate level indicates a concurrent, primary process that generates bicarbonate—specifically, a metabolic alkalosis.

5

Correlate with the clinical presentation.

The recent doubling of furosemide caused volume depletion (elevated BUN/Cr ratio of >20:1) and hypokalemia (

3.1 \text{ mEq/L}

), both of which independently generate and maintain a metabolic alkalosis.

Matching calculated lab abnormalities with clinical interventions confirms the diagnosis.

Key Concept

In mixed acid-base disorders, if the actual serum bicarbonate is significantly higher than the expected compensatory bicarbonate for a primary respiratory acidosis, a superimposed metabolic alkalosis is present.

Estimated Time:2m 0s

Question 37Question →

Paramedics bring a lethargic 45-year-old male patient to the acute care unit. According to his brother, the patient has consumed heavy amounts of alcohol daily for the past two weeks while completely abstaining from solid foods. Over the last 96 hours, he has suffered from intractable emesis. Physical examination reveals a deeply furrowed tongue, delayed capillary refill, and a loss of skin elasticity.

Laboratory evaluations reveal the following:

Parameter	Result
Sodium ( $Na^+$ )	140 mEq/L
Potassium ( $K^+$ )	3.2 mEq/L
Chloride ( $Cl^-$ )	89 mEq/L
Bicarbonate ( $HCO_3^-$ )	18 mEq/L
Blood urea nitrogen	42 mg/dL
Serum creatinine	1.6 mg/dL
Plasma glucose	70 mg/dL

Urinalysis is strongly positive for ketones.

Assume a baseline normal serum anion gap of 12 mEq/L and a normal serum bicarbonate of 24 mEq/L. What is the calculated delta ratio ( $\frac{\Delta AG}{\Delta HCO_3^-}$ ) for this patient?

Show answer & explanation

Answer: 3.5

Answer

The calculated delta ratio is 3.5.

The patient's anion gap is calculated as

140 - (89 + 18) = 33

mEq/L. The change in anion gap (

\Delta AG

) is

33 - 12 = 21

mEq/L. The change in bicarbonate (

\Delta HCO_3^-

) is

24 - 18 = 6

mEq/L. The delta ratio is

21 / 6 = 3.5

. A ratio greater than 2.0 reveals that the drop in bicarbonate is much less than expected for the degree of anion gap elevation, which is characteristic of a concurrent metabolic alkalosis (in this case, from severe volume depletion and intractable emesis superimposed on alcoholic ketoacidosis).

Step-by-Step Solution

1

Calculate the Anion Gap (AG)

33

mEq/L

To identify the presence and magnitude of a high anion gap metabolic acidosis.

2

Calculate

\Delta AG

21

mEq/L

To determine how much the anion gap has increased above the baseline of 12 mEq/L.

3

Calculate

\Delta HCO_3^-

6

mEq/L

To determine how much the bicarbonate level has decreased from the baseline of 24 mEq/L.

4

Calculate the Delta Ratio

3.5

To assess for additional hidden acid-base disorders, such as a concurrent metabolic alkalosis.

Key Concept

The delta ratio (

\Delta AG / \Delta HCO_3^-

) is used in high anion gap metabolic acidosis to identify mixed acid-base disorders; a ratio significantly greater than 2.0 indicates a concurrent metabolic alkalosis.

Question 38Question →

A 71-year-old man is evaluated in the clinic for increasing fatigue, muscle cramps, and intermittent confusion over the past week. He has a long-standing history of severe chronic obstructive pulmonary disease (COPD) and cor pulmonale. Three weeks ago, his primary care physician significantly increased his daily dose of oral furosemide to manage worsening lower extremity edema. On physical examination, the patient has a barrel-shaped chest, prolonged expiratory phase, and diminished breath sounds bilaterally. Trace pitting edema is present in the lower extremities.

Arterial blood gas on room air reveals:
pH: $7.39$
$p\text{CO}_2$ : $60$ mmHg
$p\text{O}_2$ : $58$ mmHg

Serum chemistry panel shows:
$\text{Na}^+$ : $139$ mEq/L
$\text{K}^+$ : $2.8$ mEq/L
$\text{Cl}^-$ : $92$ mEq/L
$\text{HCO}_3^-$ : $36$ mEq/L
Blood urea nitrogen: $32$ mg/dL
Creatinine: $1.4$ mg/dL

Which of the following best characterizes this patient's acid-base status?

Mixed primary respiratory acidosis and primary metabolic alkalosis

Primary respiratory acidosis with appropriate chronic metabolic compensation

Primary metabolic alkalosis with appropriate respiratory compensation

Primary respiratory acidosis and a concurrent non-anion gap metabolic acidosis

Mixed primary respiratory alkalosis and high anion gap metabolic acidosis

Show answer & explanation

Answer: Mixed primary respiratory acidosis and primary metabolic alkalosis

Answer

The patient has a mixed primary respiratory acidosis and primary metabolic alkalosis.

This patient's acid-base status is characterized by a mixed primary respiratory acidosis and primary metabolic alkalosis. His severe COPD leads to chronic

CO_2

retention, establishing a primary respiratory acidosis. However, his recent increase in furosemide has caused volume contraction and severe hypokalemia (

2.8

mEq/L), triggering independent renal bicarbonate reabsorption and generating a primary metabolic alkalosis. When calculating chronic respiratory compensation (

3.5

mEq/L

\text{HCO}_3^-

increase per

10

mmHg

p\text{CO}_2

increase), a

p\text{CO}_2

of

60

mmHg should yield a bicarbonate of roughly

31

mEq/L. His actual bicarbonate of

36

mEq/L is far higher than expected, proving the presence of the concurrent metabolic alkalosis. These two opposing primary disorders pull the pH in opposite directions, resulting in a deceptively normal pH of

7.39

.

Step-by-Step Solution

1

Evaluate the pH and identify the primary derangements.

pH is 7.39 (near normal),

p\text{CO}_2

is high at 60 mmHg (respiratory acidosis), and

\text{HCO}_3^-

is high at 36 mEq/L (metabolic alkalosis).

A normal pH despite highly abnormal

p\text{CO}_2

and

\text{HCO}_3^-

values strongly indicates the presence of a mixed, opposing acid-base disorder.

2

Assume respiratory acidosis is the primary disorder and calculate the expected chronic metabolic compensation.

Expected

\text{HCO}_3^-

increases by 3.5 mEq/L for every 10 mmHg increase in

p\text{CO}_2

. With

\Delta p\text{CO}_2

of 20, expected

\text{HCO}_3^-

=

24 + (2 \times 3.5) = 31

mEq/L.

This establishes the baseline expected bicarbonate if the patient solely had COPD-induced chronic respiratory acidosis.

3

Compare actual

\text{HCO}_3^-

to the expected value.

Actual

\text{HCO}_3^-

(36 mEq/L) is significantly higher than the expected value (31 mEq/L).

The excess bicarbonate confirms an independent, concurrent primary metabolic alkalosis, likely driven by loop diuretic use (contraction alkalosis and hypokalemia).

Key Concept

Patients with chronic respiratory acidosis (e.g., severe COPD) who are treated with loop diuretics commonly develop a superimposed primary metabolic alkalosis, resulting in highly elevated bicarbonate levels and a deceptively normal pH.

Estimated Time:1m 30s

Question 39Question →

A 56-year-old man is brought to the emergency department by paramedics after being found unresponsive in an alley. The paramedics administered intravenous naloxone en route, which resulted in a transient increase in his respiratory rate, but he remains obtunded upon arrival. The patient is unhoused and has a history of severe alcohol use disorder. His temperature is 36.1°C (97.0°F), blood pressure is 105/65 mm Hg, pulse is 92/min, and respirations are 8/min and shallow. Pulse oximetry shows 88% on room air.

Laboratory studies show the following:
Serum sodium: 142 mEq/L
Serum potassium: 4.8 mEq/L
Serum chloride: 100 mEq/L
Serum bicarbonate: 12 mEq/L
Blood urea nitrogen: 32 mg/dL
Serum creatinine: 1.7 mg/dL

Arterial blood gas analysis on room air:
pH: 7.18
$pCO_2$ : 50 mm Hg
$pO_2$ : 60 mm Hg

Based on these findings, which of the following best describes this patient's acid-base disturbance?

High anion gap metabolic acidosis and concurrent respiratory acidosis

High anion gap metabolic acidosis with appropriate respiratory compensation

High anion gap metabolic acidosis and concurrent respiratory alkalosis

Normal anion gap metabolic acidosis and concurrent respiratory acidosis

Primary respiratory acidosis with appropriate metabolic compensation

Show answer & explanation

Answer: High anion gap metabolic acidosis and concurrent respiratory acidosis

Answer

High anion gap metabolic acidosis and concurrent respiratory acidosis

The patient's laboratory results reveal an elevated anion gap of 30 mEq/L (142 - [100 + 12]), which establishes a primary high anion gap metabolic acidosis (HAGMA), likely due to a toxic alcohol ingestion or lactic acidosis. To determine if there is a secondary acid-base disorder, Winter's formula must be applied to calculate the expected respiratory compensation: expected

pCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2

. For this patient, the expected

pCO_2

is

(1.5 \times 12) + 8 = 26 \pm 2

mm Hg. The actual

pCO_2

of 50 mm Hg is significantly higher than the expected range (24-28 mm Hg). This indicates inadequate alveolar ventilation resulting in CO2 retention, thereby diagnosing a concurrent primary respiratory acidosis. This is clinically consistent with the patient's shallow respirations and low respiratory rate, likely due to central nervous system depression from an unknown intoxicant.

Step-by-Step Solution

1

Calculate the anion gap to identify the primary metabolic disturbance.

Anion gap =

142 - (100 + 12) = 30

mEq/L.

An anion gap > 12 mEq/L establishes the presence of a primary high anion gap metabolic acidosis (HAGMA).

2

Apply Winter's formula to determine the expected respiratory compensation for the metabolic acidosis.

Expected

pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = 1.5 \times 12 + 8 \pm 2 = 26 \pm 2

mm Hg.

This establishes the target range of

pCO_2

(24 to 28 mm Hg) that would be expected if the patient's respiratory system were appropriately compensating.

3

Compare the actual measured

pCO_2

to the expected

pCO_2

to identify any secondary acid-base disorder.

The actual

pCO_2

is 50 mm Hg, which is significantly higher than the expected maximum of 28 mm Hg.

A

pCO_2

higher than the expected compensated range indicates inadequate alveolar ventilation, signifying a concurrent primary respiratory acidosis. This aligns clinically with the patient's hypoventilation due to suspected intoxication.

Key Concept

Diagnosis of mixed acid-base disorders requires calculating the anion gap to identify a primary metabolic acidosis, followed by applying Winter's formula (

pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2

) to determine if the respiratory compensation is appropriate, insufficient (indicating concurrent respiratory acidosis), or excessive (indicating concurrent respiratory alkalosis).

Estimated Time:1m 30s

Question 40Question →

A 32-year-old woman presents to the ambulatory care clinic reporting four days of frequent, large-volume, watery stools after returning from an international trip. During the interview, she is noticeably distressed and pacing the room. She states she is terrified of acquiring a severe parasitic infection and has been breathing rapidly for the past hour. Physical examination reveals dry mucous membranes and hyperactive bowel sounds. Vital signs include a heart rate of $112/\text{min}$ , blood pressure of $105/65\ \text{mm Hg}$ , and respiratory rate of $30/\text{min}$ . Laboratory results are as follows:

Serum sodium: $138\ \text{mEq/L}$
Serum potassium: $3.4\ \text{mEq/L}$
Serum chloride: $116\ \text{mEq/L}$
Serum bicarbonate: $12\ \text{mEq/L}$

Arterial blood gas (room air):
pH: $7.38$
$pCO_2$ : $21\ \text{mm Hg}$

Which of the following best characterizes this patient's current acid-base status?

Mixed normal anion gap metabolic acidosis and primary respiratory alkalosis

Appropriately compensated normal anion gap metabolic acidosis

Mixed high anion gap metabolic acidosis and primary respiratory alkalosis

Mixed normal anion gap metabolic acidosis and primary respiratory acidosis

Primary respiratory alkalosis with appropriate metabolic compensation

Show answer & explanation

Answer: Mixed normal anion gap metabolic acidosis and primary respiratory alkalosis

Answer

Mixed normal anion gap metabolic acidosis and primary respiratory alkalosis

The correct answer accurately synthesizes all steps of the acid-base algorithm. The pH is 7.38 (primary acidosis). The low bicarbonate (12 mEq/L) identifies the acidosis as metabolic. The anion gap is 10 mEq/L, defining it as a normal anion gap (hyperchloremic) metabolic acidosis, consistent with bicarbonate loss from diarrhea. Finally, applying Winter's formula yields an expected pCO2 of 24 to 28 mm Hg. The patient's actual pCO2 of 21 mm Hg is significantly lower than this range, indicating she is hyperventilating beyond what is required for compensation. This confirms a superimposed primary respiratory alkalosis, clinically correlating with her acute distress and tachypnea.

Step-by-Step Solution

1

Determine the primary acid-base disorder by evaluating the pH.

The pH is

7.38

, which is less than

7.40

(acidemia).

The direction of the pH deviation from normal (7.40) always indicates the primary disorder. Therefore, the primary process is an acidosis.

2

Examine the serum bicarbonate and

pCO_2

to identify the source of the acidosis.

Serum bicarbonate is low (

12\ \text{mEq/L}

) and

pCO_2

is low (

21\ \text{mm Hg}

).

A primary metabolic acidosis is characterized by a low bicarbonate level. The decreased

pCO_2

indicates a respiratory response.

3

Calculate the serum anion gap.

\text{Anion Gap} = \text{Na} - (\text{Cl} + \text{HCO}_3) = 138 - (116 + 12) = 10\ \text{mEq/L}

.

Determining whether the metabolic acidosis is an anion gap or non-anion gap process narrows the differential. An AG of

10

is normal (reference range

8-12\ \text{mEq/L}

). Severe diarrhea typically causes a normal anion gap metabolic acidosis due to direct bicarbonate loss.

4

Determine if the respiratory compensation is appropriate using Winter's formula.

\text{Expected } pCO_2 = (1.5 \times 12) + 8 \pm 2 = 18 + 8 \pm 2 = 26 \pm 2

(range

24-28\ \text{mm Hg}

).

Winter's formula calculates the expected respiratory compensation for a given degree of metabolic acidosis to identify superimposed respiratory disorders.

5

Compare the actual

pCO_2

to the expected

pCO_2

.

The actual

pCO_2

(

21\ \text{mm Hg}

) is lower than the expected range (

24-28\ \text{mm Hg}

).

Because the patient is blowing off more

CO_2

than expected for simple compensation, there is a coexisting primary respiratory alkalosis, likely driven by her acute anxiety and hyperventilation.

Key Concept

Diagnosis of mixed acid-base disorders requires sequential analysis: determining the primary disorder via pH, calculating the anion gap, and verifying respiratory compensation using Winter's formula.

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