Nephrology Practice Questions — USMLE Step 2 CK — Page 16

Question 301Question →

Paramedics bring a 52-year-old unhoused man to the emergency department after he was found unresponsive in an alleyway during the winter. He has a known history of alcohol use disorder. On examination, he is obtunded and tachypneic.

Vital signs include a heart rate of $105/\text{min}$ , blood pressure of $115/75 \text{ mmHg}$ , and respiratory rate of $24/\text{min}$ .

Laboratory studies reveal the following:

Test	Result
Sodium	$140 \text{ mEq/L}$
Potassium	$4.5 \text{ mEq/L}$
Chloride	$102 \text{ mEq/L}$
Bicarbonate	$14 \text{ mEq/L}$
Blood urea nitrogen (BUN)	$14 \text{ mg/dL}$
Creatinine	$1.5 \text{ mg/dL}$
Glucose	$90 \text{ mg/dL}$
Serum measured osmolality	$325 \text{ mOsm/kg}$
Blood ethanol level	$0 \text{ mg/dL}$

Arterial blood gas on room air shows a pH of $7.25$ , $\text{pCO}_2$ of $28 \text{ mmHg}$ , and $\text{pO}_2$ of $95 \text{ mmHg}$ .

Based on the laboratory results provided, what is this patient's calculated serum osmolar gap in $\text{mOsm/kg}$ ?

Show answer & explanation

Answer: 35

Answer

The calculated serum osmolar gap is

35 \text{ mOsm/kg}

.

The serum osmolar gap is calculated by subtracting the calculated serum osmolality from the directly measured serum osmolality. The formula for calculated serum osmolality is

2 \times [\text{Na}^+] + \frac{\text{Glucose}}{18} + \frac{\text{BUN}}{2.8}

. For this patient, the calculated osmolality is

2(140) + \frac{90}{18} + \frac{14}{2.8} = 290 \text{ mOsm/kg}

. Subtracting this from the measured osmolality of

325 \text{ mOsm/kg}

yields an osmolar gap of

35 \text{ mOsm/kg}

. An elevated osmolar gap (

> 10 \text{ mOsm/kg}

) in the setting of a high anion gap metabolic acidosis strongly suggests intoxication with an unmeasured osmotically active substance, such as methanol or ethylene glycol.

Step-by-Step Solution

1

Recognize the need to calculate the osmolar gap.

The patient has an unexplained high anion gap metabolic acidosis and altered mental status, raising suspicion for toxic alcohol ingestion.

Calculating the osmolar gap is an essential diagnostic step when toxic alcohol poisoning (like ethylene glycol or methanol) is suspected.

2

Calculate the expected serum osmolality.

Calculated osmolality =

2(140) + \frac{90}{18} + \frac{14}{2.8} = 280 + 5 + 5 = 290 \text{ mOsm/kg}

.

The standard formula estimates the contribution of the major serum osmoles: sodium (and its associated anions), glucose, and urea.

3

Calculate the osmolar gap.

Osmolar gap =

325 \text{ mOsm/kg} - 290 \text{ mOsm/kg} = 35 \text{ mOsm/kg}

.

The osmolar gap is defined as the measured serum osmolality minus the calculated serum osmolality. A normal gap is typically less than

10 \text{ mOsm/kg}

.

Key Concept

Calculation of the serum osmolar gap is used to detect the presence of unmeasured, osmotically active substances (such as toxic alcohols) in the blood.

Question 302Question →

A 68-year-old man is brought to the emergency department due to worsening shortness of breath, a productive cough, and progressive lethargy over the past 3 days. He has a history of severe chronic obstructive pulmonary disease (COPD) and heart failure with a reduced ejection fraction. One week ago, his primary care physician doubled his daily dose of oral furosemide due to increased lower extremity edema. On physical examination, his temperature is 37.2°C (99.0°F), blood pressure is 110/70 mm Hg, pulse is 98/min, and respiratory rate is 14/min. He is somnolent but arousable. Lung auscultation reveals diffuse expiratory wheezes and prolonged expiration. There is no lower extremity edema.

Arterial blood gas on room air:
pH: 7.37
$pCO_2$ : 80 mm Hg
$pO_2$ : 55 mm Hg

Serum chemistry:
Sodium: 137 mEq/L
Potassium: 2.9 mEq/L
Chloride: 80 mEq/L
Bicarbonate: 45 mEq/L

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

Mixed respiratory acidosis and metabolic alkalosis

Fully compensated chronic respiratory acidosis

Mixed metabolic alkalosis and respiratory alkalosis

Compensated metabolic alkalosis

Mixed respiratory acidosis and high anion gap metabolic acidosis

Show answer & explanation

Answer: Mixed respiratory acidosis and metabolic alkalosis

Answer

Mixed respiratory acidosis and metabolic alkalosis

This patient presents with a pH of 7.37 and a

pCO_2

of 80 mm Hg, indicating a primary respiratory acidosis driven by his severe COPD exacerbation. To determine whether this is a simple or mixed disorder, one must calculate the expected renal compensation. In chronic respiratory acidosis, the kidneys retain bicarbonate to compensate, typically increasing serum

HCO_3^-

by roughly 4 mEq/L for every 10 mm Hg increase in

pCO_2

. Since this patient's

pCO_2

is 40 mm Hg above normal (80 - 40), his maximal expected

HCO_3^-

should be approximately

24 + (4 \times 4) = 40

mEq/L. His measured

HCO_3^-

of 45 mEq/L significantly exceeds this physiological limit. This reveals a second, independent process that is actively raising the bicarbonate: a primary metabolic alkalosis. This secondary disorder was precipitated by the recent doubling of his furosemide dose, which led to volume depletion, chloride loss, and severe hypokalemia (contraction alkalosis).

Step-by-Step Solution

1

Determine the primary acid-base disorder by looking at the pH and pCO2.

The pH is 7.37 (< 7.40), indicating acidemia. The

pCO_2

is elevated at 80 mm Hg. Because the

pCO_2

movement matches the acidic pH, the primary disorder is a respiratory acidosis.

Establishing the primary disorder dictates which compensation formula to use.

2

Calculate the expected metabolic compensation for the respiratory acidosis.

This patient has severe COPD, implying a chronic state. For chronic respiratory acidosis, serum

HCO_3^-

increases by approximately 3.5 to 4 mEq/L for every 10 mm Hg increase in

pCO_2

above 40 mm Hg.

Kidneys compensate for chronic hypercapnia by retaining bicarbonate, but this compensation has predictable physiological limits.

3

Compare the patient's actual bicarbonate to the expected calculated bicarbonate.

The

pCO_2

is 40 mm Hg above normal (80 - 40). The expected

HCO_3^-

increase is

4 \times 4 = 16

mEq/L. Thus, maximum expected

HCO_3^-

is

24 + 16 = 40

mEq/L. The patient's actual

HCO_3^-

is 45 mEq/L.

An actual

HCO_3^-

higher than the maximum expected compensation indicates an additional, independent primary metabolic alkalosis.

Key Concept

Identifying mixed acid-base disorders by applying compensation rules for chronic respiratory acidosis.

Question 303Question →

Following 4 days of profuse, watery diarrhea, a 70-year-old man with a long-standing history of severe chronic obstructive pulmonary disease presents to the emergency department for evaluation of extreme weakness. He reports having 8 to 10 bowel movements daily since attending a family picnic. His home medications include a long-acting muscarinic antagonist and an inhaled corticosteroid.

Vital signs show a temperature of $37.4^\circ\text{C}$ ( $99.3^\circ\text{F}$ ), blood pressure of 102/64 mm Hg, pulse of 115/min, and respiratory rate of 14/min. Physical examination reveals dry mucous membranes, decreased skin turgor, and diffuse expiratory wheezes bilaterally.

Laboratory studies are obtained:

Parameter	Value
Sodium	$140 \text{ mEq/L}$
Potassium	$2.9 \text{ mEq/L}$
Chloride	$114 \text{ mEq/L}$
Bicarbonate	$16 \text{ mEq/L}$
Blood urea nitrogen	$35 \text{ mg/dL}$
Creatinine	$1.4 \text{ mg/dL}$

Arterial blood gas on room air:

Parameter	Value
pH	$7.15$
$PaCO_2$	$48 \text{ mm Hg}$
$PaO_2$	$62 \text{ mm Hg}$

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

Normal anion gap metabolic acidosis and respiratory acidosis

Normal anion gap metabolic acidosis with adequate respiratory compensation

High anion gap metabolic acidosis and respiratory acidosis

Normal anion gap metabolic acidosis and respiratory alkalosis

Chronic respiratory acidosis with fully compensated metabolic alkalosis

Show answer & explanation

Answer: Normal anion gap metabolic acidosis and respiratory acidosis

Answer

Normal anion gap metabolic acidosis and respiratory acidosis

The patient's severe diarrhea leads to direct gastrointestinal loss of bicarbonate, causing a primary normal anion gap metabolic acidosis (NAGMA). In a healthy individual, the expected physiologic response is hyperventilation to blow off

CO_2

. Using Winter's formula (

PaCO_2 = 1.5 \times [\text{HCO}_3^-] + 8 \pm 2

), the expected

PaCO_2

for this patient should be between

30

and

34 \text{ mm Hg}

. However, his actual

PaCO_2

is

48 \text{ mm Hg}

. Because his actual

PaCO_2

is much higher than the expected compensatory range, he has a superimposed respiratory acidosis. This occurs because his severe underlying COPD prevents him from adequately increasing his minute ventilation to compensate for the metabolic acidemia.

Step-by-Step Solution

1

Determine the primary disturbance by looking at pH and bicarbonate.

The pH is

7.15

(acidemia) and bicarbonate is

16 \text{ mEq/L}

(low). This establishes a primary metabolic acidosis.

Acidemia with a corresponding decrease in serum bicarbonate indicates that a metabolic acidosis is the primary driving process.

2

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

\text{AG} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (114 + 16) = 10 \text{ mEq/L}

.

An AG of

10 \text{ mEq/L}

falls within the normal range (

8-12 \text{ mEq/L}

), identifying a normal anion gap metabolic acidosis (NAGMA).

3

Calculate the expected respiratory compensation using Winter's formula.

Expected

PaCO_2 = (1.5 \times 16) + 8 \pm 2 = 32 \pm 2 \text{ mm Hg}

(Range:

30-34 \text{ mm Hg}

).

Winter's formula determines if the lungs are appropriately blowing off

CO_2

to compensate for the primary metabolic acidosis.

4

Compare the actual

PaCO_2

to the expected

PaCO_2

range.

The actual

PaCO_2

is

48 \text{ mm Hg}

, which is significantly higher than the expected

30-34 \text{ mm Hg}

.

Because the actual

PaCO_2

is higher than expected, there is a concomitant, uncompensated primary respiratory acidosis.

Key Concept

Patients with underlying chronic lung disease (like COPD) may fail to mount an appropriate respiratory compensation (hyperventilation) to a primary metabolic acidosis, resulting in a mixed metabolic and respiratory acidosis.

Question 304Question →

Match each clinical presentation and urinalysis finding with the most likely underlying renal pathology.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Contrast-induced injury with muddy brown casts matches Acute tubular necrosis. Nafcillin-induced injury with WBC casts matches Acute interstitial nephritis. Post-impetigo injury with RBC casts matches Glomerulonephritis. Diabetic nephropathy with fatty casts matches Nephrotic syndrome.

The correct pairings rely on identifying the hallmark urinalysis findings and clinical risk factors for each renal pathology. Acute tubular necrosis is characterized by muddy brown casts following an ischemic or nephrotoxic insult. Acute interstitial nephritis is indicated by white blood cell casts and eosinophils following exposure to an offending drug. Glomerulonephritis is diagnosed by red blood cell casts in the setting of nephritic syndrome symptoms. Nephrotic syndrome presents with heavy proteinuria, hypoalbuminemia, and fatty casts.

Step-by-Step Solution

1

Analyze the first patient's history of recent radiocontrast administration and the presence of muddy brown granular casts.

Identify this pattern as characteristic of acute tubular necrosis (ATN).

Contrast is a known nephrotoxin that damages tubular cells, which slough off and form muddy brown casts.

2

Evaluate the second patient's presentation of fever, rash, and new medication exposure (nafcillin) alongside white blood cell casts.

Identify this pattern as acute interstitial nephritis (AIN).

AIN is an allergic inflammatory reaction in the kidney interstitium, often drug-induced, characterized by WBCs and eosinophils.

3

Assess the third patient's post-infectious presentation (impetigo) featuring hypertension, edema, and red blood cell casts.

Identify this pattern as glomerulonephritis.

RBC casts form when red blood cells leak through a damaged glomerular basement membrane, a hallmark of acute nephritic syndrome.

4

Review the fourth patient's long-standing diabetes, profound hypoalbuminemia, and fatty casts in the urine.

Identify this pattern as nephrotic syndrome.

Severe proteinuria leads to hypoalbuminemia and a compensatory increase in hepatic lipid synthesis, resulting in lipiduria (fatty casts).

Key Concept

Urinalysis sediment interpretation in the differential diagnosis of acute kidney injury and proteinuric states.

Question 305Question →

A $58$ -year-old woman comes to the clinic for a routine evaluation. She has a $15$ -year history of type $2$ diabetes mellitus complicated by peripheral neuropathy. The patient reports feeling generally well and denies nausea, vomiting, diarrhea, or shortness of breath. Her current medications include metformin and amlodipine. Blood pressure is $138/84 \text{ mmHg}$ and pulse is $76/\text{min}$ . Physical examination shows decreased sensation to monofilament testing on the plantar surfaces of both feet but is otherwise unremarkable.

Laboratory studies reveal the following:
Serum sodium: $140 \text{ mEq/L}$
Serum potassium: $5.6 \text{ mEq/L}$
Serum chloride: $112 \text{ mEq/L}$
Serum bicarbonate: $16 \text{ mEq/L}$
Blood urea nitrogen: $32 \text{ mg/dL}$
Serum creatinine: $1.9 \text{ mg/dL}$
Blood glucose: $148 \text{ mg/dL}$
Urine pH: $5.2$

Which of the following best describes the primary underlying pathogenesis of this patient's acid-base and electrolyte findings?

Reduced production of renin by the juxtaglomerular apparatus

Impaired hydrogen ion secretion in the distal tubule

Decreased bicarbonate reabsorption in the proximal tubule

Loss of bicarbonate-rich fluid from the lower gastrointestinal tract

Retention of unmeasured endogenous anions

Show answer & explanation

Answer: Reduced production of renin by the juxtaglomerular apparatus

Answer

Reduced production of renin by the juxtaglomerular apparatus

The correct answer identifies the pathogenesis of Type 4 renal tubular acidosis (RTA). The patient's anion gap is normal (

12 \text{ mEq/L}

), confirming a normal anion gap metabolic acidosis. The concurrent hyperkalemia strongly suggests Type 4 RTA. Patients with long-standing diabetes mellitus frequently develop damage to the juxtaglomerular apparatus, resulting in decreased renin secretion. This leads to reduced aldosterone production (hyporeninemic hypoaldosteronism), which subsequently impairs both potassium and hydrogen ion excretion in the distal nephron.

Step-by-Step Solution

1

Calculate the serum anion gap using the formula:

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

.

140 - (112 + 16) = 12 \text{ mEq/L}

.

Determining the anion gap is the essential first step in evaluating any metabolic acidosis.

2

Evaluate the patient's potassium level and urine pH in the context of the normal anion gap metabolic acidosis (NAGMA).

The patient has hyperkalemia (

5.6 \text{ mEq/L}

) and an appropriately acidic urine (pH

5.2

).

Potassium levels help differentiate the causes of NAGMA. Hyperkalemia points toward Type 4 RTA, while hypokalemia points toward Type 1 RTA, Type 2 RTA, or diarrhea.

3

Correlate the laboratory findings with the patient's medical history.

The patient has long-standing diabetes mellitus with nephropathy (elevated creatinine).

Diabetic nephropathy classically damages the juxtaglomerular apparatus, leading to hyporeninemic hypoaldosteronism (Type 4 RTA), which explains the hyperkalemia and acidosis.

Key Concept

Type 4 Renal Tubular Acidosis (Hyporeninemic Hypoaldosteronism)

Question 306Question →

A 35-year-old woman with a history of systemic lupus erythematosus presents to the clinic with progressive muscle weakness and fatigue over the past 2 weeks. She denies any recent illnesses, diarrhea, or vomiting. Current medications include hydroxychloroquine. Vital signs are within normal limits. Laboratory testing reveals the following:

Test	Value
Serum sodium	$138 \text{ mEq/L}$
Serum potassium	$2.9 \text{ mEq/L}$
Serum chloride	$114 \text{ mEq/L}$
Serum bicarbonate	$14 \text{ mEq/L}$
Serum creatinine	$0.8 \text{ mg/dL}$
Urine sodium	$38 \text{ mEq/L}$
Urine potassium	$22 \text{ mEq/L}$
Urine chloride	$15 \text{ mEq/L}$
Urine pH	$6.8$

What is this patient's calculated urine anion gap?

Show answer & explanation

Answer: 45

Answer

45

The correct answer is 45.

The urine anion gap (UAG) is calculated using the formula:

UAG = Na^+_{urine} + K^+_{urine} - Cl^-_{urine}

Plugging in the values from the vignette:

UAG = 38 \text{ mEq/L (sodium)} + 22 \text{ mEq/L (potassium)} - 15 \text{ mEq/L (chloride)} = 45

In a patient with normal anion gap metabolic acidosis (NAGMA), the UAG helps distinguish between extrarenal and renal causes of bicarbonate loss. A positive UAG (

>0

) suggests impaired renal ammonium (

NH_4^+

) excretion, pointing to a renal etiology such as renal tubular acidosis (RTA). In this patient with systemic lupus erythematosus, hypokalemia, and a urine pH

> 5.5

, the presentation is highly consistent with Type 1 (distal) RTA.

In contrast, a negative UAG (

<-10

) would suggest appropriately increased ammonium excretion, pointing to an extrarenal cause like gastrointestinal bicarbonate loss from profuse diarrhea.

Step-by-Step Solution

1

Calculate the serum anion gap to classify the acid-base disturbance.

Serum anion gap =

138 - (114 + 14) = 10 \text{ mEq/L}

.

This confirms the presence of a normal anion gap metabolic acidosis (NAGMA), which indicates the need for a urine anion gap evaluation.

2

Select the correct formula for the urine anion gap (UAG).

The formula is

UAG = Na^+_{urine} + K^+_{urine} - Cl^-_{urine}

.

The UAG estimates the unmeasured urinary cations (primarily ammonium,

NH_4^+

) and anions to help differentiate renal versus gastrointestinal causes of NAGMA.

3

Substitute the patient's urine electrolyte values into the formula.

UAG = 38 + 22 - 15 = 45

.

This yields a positive UAG, indicating impaired renal ammonium excretion, consistent with Type 1 distal renal tubular acidosis.

Key Concept

The urine anion gap (

UAG = Na^+_{urine} + K^+_{urine} - Cl^-_{urine}

) is used to evaluate normal anion gap metabolic acidosis; a positive value indicates a renal etiology (like renal tubular acidosis), whereas a negative value indicates extrarenal bicarbonate loss (like diarrhea).

Estimated Time:1m 0s

Question 307Question →

Emergency medical responders bring a 19-year-old man to the hospital after he was discovered unresponsive in his university dormitory. His roommate reports that the patient has type 1 diabetes mellitus and has been struggling with depression, recently experimenting with unprescribed oxycodone. On arrival, his temperature is 36.8°C (98.2°F), blood pressure is 105/65 mm Hg, pulse is 114/min, and respirations are 8/min. Physical examination reveals a somnolent patient with shallow breathing, dry mucous membranes, and pinpoint pupils.

Laboratory results are as follows:
Serum sodium: 140 mEq/L
Serum potassium: 4.8 mEq/L
Serum chloride: 102 mEq/L
Serum bicarbonate: 12 mEq/L
Blood glucose: 480 mg/dL
Blood urea nitrogen: 28 mg/dL
Serum creatinine: 1.3 mg/dL

Arterial blood gas analysis on room air:
pH: 7.12
PaCO2: 38 mm Hg
PaO2: 60 mm Hg

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

High anion gap metabolic acidosis and respiratory acidosis

High anion gap metabolic acidosis with appropriate respiratory compensation

High anion gap metabolic acidosis and respiratory alkalosis

High anion gap metabolic acidosis and normal anion gap metabolic acidosis

Respiratory acidosis with appropriate metabolic compensation

Show answer & explanation

Answer: High anion gap metabolic acidosis and respiratory acidosis

Answer

The correct answer is high anion gap metabolic acidosis and respiratory acidosis.

This patient has diabetic ketoacidosis resulting in a primary high anion gap metabolic acidosis, as evidenced by an anion gap of 26 mEq/L. According to Winter's formula, the expected PaCO2 should be between 24 and 28 mm Hg. The patient's actual PaCO2 is 38 mm Hg. Because the actual PaCO2 is higher than expected, a superimposed respiratory acidosis is present, which is clinically consistent with opiate toxicity (pinpoint pupils, hypoventilation).

Step-by-Step Solution

1

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

The pH is 7.12 (acidemia) and the serum bicarbonate is 12 mEq/L. Both are decreased, indicating a primary metabolic acidosis.

Determining the primary disorder is always the first step in acid-base analysis.

2

Calculate the serum anion gap to classify the metabolic acidosis.

\text{Anion Gap} = \text{Na} - (\text{Cl} + \text{HCO}_3^-) = 140 - (102 + 12) = 26

mEq/L. This is greater than the normal value of 12 mEq/L, establishing a high anion gap metabolic acidosis (HAGMA).

All metabolic acidoses must be categorized as high or normal anion gap to narrow the differential diagnosis (e.g., DKA causing HAGMA).

3

Apply Winter's formula to determine if the respiratory compensation is adequate.

\text{Expected PaCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2 = (1.5 \times 12) + 8 = 26 \pm 2

mm Hg. The expected range is 24 to 28 mm Hg.

Winter's formula calculates the expected PaCO2 for a given metabolic acidosis. Discrepancies indicate a secondary respiratory disorder.

4

Compare the actual PaCO2 to the expected PaCO2.

The patient's actual PaCO2 is 38 mm Hg, which is significantly higher than the expected maximum of 28 mm Hg. This indicates a superimposed respiratory acidosis.

A higher-than-expected PaCO2 means the lungs are failing to clear CO2 adequately, consistent with the patient's opiate-induced respiratory depression (respirations 8/min).

5

Calculate the delta-delta ratio to rule out additional metabolic disturbances.

\Delta\text{AG} = 26 - 12 = 14

.

\Delta\text{HCO}_3^- = 24 - 12 = 12

. Ratio =

14 / 12 = 1.16

. A ratio between 1.0 and 2.0 confirms a pure HAGMA.

This confirms there is no hidden normal anion gap metabolic acidosis or concurrent metabolic alkalosis.

Key Concept

In patients with severe metabolic acidosis, a 'normal' PaCO2 value (e.g., 35-45 mm Hg) is highly abnormal and indicates a failure of appropriate respiratory compensation, representing a superimposed respiratory acidosis.

Question 308Question →

A diverse group of patients presents to the nephrology clinic with various renal abnormalities. Match each patient's clinical presentation with the most likely underlying glomerular pathology.

Click a left item, then click its matching right item

Show answer & explanation

Answer

The lung mass with thrombosis matches Membranous nephropathy; the synpharyngitic hematuria matches IgA nephropathy; the Hepatitis C with purpura matches Membranoproliferative glomerulonephritis; and the HIV patient with nephrotic syndrome matches Focal segmental glomerulosclerosis.

Glomerular diseases often present similarly with edema and urinalysis abnormalities. Accurate diagnosis relies heavily on recognizing classic associations: Membranous nephropathy with solid tumors and thrombosis, IgA nephropathy with concurrent respiratory infections, Membranoproliferative glomerulonephritis with Hepatitis C, and Focal segmental glomerulosclerosis with HIV and heroin use.

Step-by-Step Solution

1

Analyze the presentation of the 55-year-old man with a lung mass.

Identify a nephrotic syndrome (heavy proteinuria, edema) complicated by thrombosis in the setting of a solid malignancy.

Solid tumors and high thrombotic risk (due to loss of antithrombin III) strongly point to membranous nephropathy.

2

Analyze the presentation of the 28-year-old man with episodic hematuria.

Identify recurrent nephritic syndrome occurring concurrently with a viral upper respiratory infection.

Concurrent (synpharyngitic) onset of hematuria is the hallmark of IgA nephropathy.

3

Analyze the presentation of the 62-year-old woman with Hepatitis C.

Identify a nephritic syndrome (RBC casts) combined with systemic signs of cryoglobulinemia (purpura, arthralgias).

Hepatitis C and cryoglobulinemia are classically linked to membranoproliferative glomerulonephritis.

4

Analyze the presentation of the 45-year-old man with HIV.

Identify massive proteinuria and declining renal function in an immunocompromised patient.

HIV-associated nephropathy is a well-known, rapidly progressive variant of focal segmental glomerulosclerosis.

Key Concept

Differentiating glomerular diseases based on unique clinical associations and patient demographics.

Estimated Time:1m 30s

Question 309Question →

A 68-year-old man is evaluated in the intensive care unit for decreased urine output. Three days ago, he was admitted with severe community-acquired pneumonia and septic shock, which required aggressive intravenous fluid resuscitation and continuous norepinephrine infusion. Over the past 24 hours, his urine output has decreased to 250 mL. His blood pressure is 105/65 mm Hg, pulse is 98/min, and respirations are 18/min. Physical examination reveals warm extremities and trace bilateral pitting edema. Laboratory studies are obtained:

Parameter	Value
Serum sodium	140 mEq/L
Serum potassium	4.8 mEq/L
Serum chloride	102 mEq/L
Serum bicarbonate	20 mEq/L
Blood urea nitrogen	42 mg/dL
Serum creatinine	2.8 mg/dL
Urine sodium	50 mEq/L
Urine creatinine	25 mg/dL

Based on these findings, what is this patient's calculated fractional excretion of sodium (in percentage)?

Show answer & explanation

Answer: 4

Answer

4

The fractional excretion of sodium (FeNa) is calculated using the formula

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

. By substituting the patient's laboratory values (

U_{Na}

= 50 mEq/L,

P_{Cr}

= 2.8 mg/dL,

P_{Na}

= 140 mEq/L,

U_{Cr}

= 25 mg/dL) into the equation, the result is

\frac{50 \times 2.8}{140 \times 25} \times 100

, which equals 4%. A FeNa > 2% in the setting of oliguric acute kidney injury strongly suggests intrinsic damage, such as acute tubular necrosis, which aligns with this patient's history of prolonged septic shock and ischemic injury.

Step-by-Step Solution

1

Extract the necessary laboratory values from the clinical vignette.

U_{Na} = 50

mEq/L,

P_{Cr} = 2.8

mg/dL,

P_{Na} = 140

mEq/L, and

U_{Cr} = 25

mg/dL.

These variables are required to calculate the fractional excretion of sodium (FeNa).

2

Apply the FeNa equation.

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

This formula normalizes sodium excretion to the glomerular filtration rate, helping differentiate prerenal azotemia from intrinsic acute kidney injury.

3

Perform the calculation with the extracted values.

\frac{50 \times 2.8}{140 \times 25} \times 100 = \frac{140}{3500} \times 100 = 0.04 \times 100 = 4\%

To obtain the final FeNa percentage. A FeNa > 2% strongly indicates acute tubular necrosis (ATN).

Key Concept

Calculation and interpretation of the Fractional Excretion of Sodium (FeNa) in acute kidney injury.

Question 310Question →

A 52-year-old woman is evaluated on postoperative day 4 following a small bowel resection for severe intestinal strictures. Since the surgery, she has been receiving continuous low-wall suction via a nasogastric tube, which has drained 1.5 to 2 liters of bilious fluid daily. Over the last 12 hours, she has developed significant weakness, muscle cramps, and dizziness upon sitting up. Her temperature is 37.1°C (98.8°F), blood pressure is 96/60 mm Hg, heart rate is 110/min, and respiratory rate is 10/min. Physical examination reveals dry mucous membranes, flat jugular veins, and decreased skin turgor. Laboratory studies show the following:

- Serum sodium: 136 mEq/L
- Serum potassium: 2.8 mEq/L
- Serum chloride: 88 mEq/L
- Serum bicarbonate: 36 mEq/L
- Blood urea nitrogen: 42 mg/dL
- Serum creatinine: 1.6 mg/dL

Arterial blood gas (room air):
- pH: 7.52
- pCO $_2$ : 49 mm Hg
- pO $_2$ : 86 mm Hg

Urine chloride is 8 mEq/L. Which of the following is the most appropriate next step in management?

Intravenous administration of 0.9% sodium chloride

Intubation and mechanical ventilation

Intravenous administration of acetazolamide

Intravenous administration of 5% dextrose in water

Initiation of broad-spectrum antibiotics

Show answer & explanation

Answer: Intravenous administration of 0.9% sodium chloride

Answer

The most appropriate next step is intravenous administration of 0.9% sodium chloride to correct the volume deficit and provide the necessary chloride to reverse the alkalosis.

The correct answer is intravenous administration of 0.9% sodium chloride. This patient has a severe metabolic alkalosis secondary to gastric acid (HCl) and volume loss from continuous nasogastric suctioning. The resulting volume depletion activates the renin-angiotensin-aldosterone system (RAAS), which promotes sodium reabsorption in the distal tubule at the expense of potassium and hydrogen ion excretion (perpetuating the alkalosis). Because the primary drivers are volume and chloride depletion, this is termed a 'saline-responsive' metabolic alkalosis, confirmed by a urine chloride level of < 20 mEq/L. Administering isotonic saline expands the intravascular space, deactivates RAAS, and provides the chloride necessary for the kidneys to excrete excess bicarbonate.

Step-by-Step Solution

1

Analyze the acid-base status on the arterial blood gas.

The pH is 7.52 (alkalemia). The elevated serum bicarbonate (36 mEq/L) identifies this as a primary metabolic alkalosis.

Establishing the primary acid-base disorder is required to interpret the compensatory mechanisms.

2

Calculate expected respiratory compensation for the metabolic alkalosis.

Expected pCO

_2

=

40 + 0.7 \times (36 - 24) = 48.4

mm Hg. The measured pCO

_2

of 49 mm Hg represents perfect physiological compensation.

This confirms there is no superimposed primary respiratory disorder, and the hypoventilation is appropriately compensatory.

3

Determine the etiology of the metabolic alkalosis using patient history and urine studies.

NG suction causes massive loss of H

^+

and Cl

^-

(gastric acid) along with fluid. The low urine chloride (< 20 mEq/L) and elevated BUN/Cr ratio (> 20:1) confirm severe volume depletion.

Differentiating between saline-responsive (urine Cl < 20) and saline-resistant (urine Cl > 20) metabolic alkalosis dictates the treatment.

4

Select the optimal therapeutic intervention.

Volume resuscitation with 0.9% sodium chloride (normal saline).

Isotonic saline simultaneously expands the intravascular volume (turning off aldosterone) and replaces the depleted chloride, allowing the kidneys to excrete bicarbonate.

Key Concept

Patients with significant upper gastrointestinal fluid losses (via vomiting or nasogastric suction) develop a hypochloremic, hypokalemic metabolic alkalosis. Because this is a 'saline-responsive' alkalosis (urine Cl < 20 mEq/L), the definitive treatment is restoration of intravascular volume and chloride with intravenous 0.9% sodium chloride.

Estimated Time:1m 30s

Question 311Question →

A 26-year-old man is brought to the emergency department due to intractable, non-bilious vomiting for the past 4 days. He has type 1 diabetes mellitus and reports running out of his insulin prescription three days ago. He has been unable to tolerate any oral intake. He appears lethargic with dry mucous membranes. Vital signs show a heart rate of 118/min and blood pressure of 98/60 mm Hg.

Serum chemistry results are as follows:
$Na^+$ : 142 mEq/L
$K^+$ : 3.3 mEq/L
$Cl^-$ : 90 mEq/L
$HCO_3^-$ : 24 mEq/L
Blood urea nitrogen: 35 mg/dL
Creatinine: 1.4 mg/dL
Glucose: 520 mg/dL

Serum ketones are positive. Arterial blood gas (on room air) shows:
pH: 7.42
$pCO_2$ : 38 mm Hg

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

Mixed high anion gap metabolic acidosis and metabolic alkalosis

Isolated high anion gap metabolic acidosis with appropriate respiratory compensation

Normal acid-base status with an artifactual anion gap elevation

Primary respiratory alkalosis with appropriate metabolic compensation

Mixed normal anion gap metabolic acidosis and metabolic alkalosis

Show answer & explanation

Answer: Mixed high anion gap metabolic acidosis and metabolic alkalosis

Answer

Mixed high anion gap metabolic acidosis and metabolic alkalosis

The correct interpretation involves identifying two independent metabolic processes that are neutralizing each other's effect on the overall pH. The patient's lack of insulin led to Diabetic Ketoacidosis (creating a High Anion Gap Metabolic Acidosis), which would normally drastically lower the

HCO_3^-

and pH. Simultaneously, his intractable vomiting caused a severe loss of gastric hydrochloric acid (creating a Metabolic Alkalosis), which raises the

HCO_3^-

and pH. The anion gap is unequivocally elevated at 28 mEq/L. By calculating the delta gap (28 - 12 = 16), we find that the expected bicarbonate should be around 8 mEq/L. Because the actual bicarbonate is 24 mEq/L, there is a large concurrent metabolic alkalosis present.

Step-by-Step Solution

1

Calculate the serum anion gap (AG) to screen for hidden acid-base disorders despite the normal pH.

AG = Na^+ - (Cl^- + HCO_3^-) = 142 - (90 + 24) = 28

mEq/L.

An elevated AG (normal is ~12 mEq/L) definitively identifies the presence of a High Anion Gap Metabolic Acidosis (HAGMA), which in this case is diabetic ketoacidosis.

2

Calculate the delta gap (

\Delta AG

) to determine if there are additional metabolic derangements.

\Delta AG = \text{Patient's AG} - \text{Normal AG} = 28 - 12 = 16

mEq/L.

For every 1 mEq/L rise in the anion gap, the serum

HCO_3^-

should drop by approximately 1 mEq/L.

3

Calculate the expected serum bicarbonate if HAGMA were the only disturbance.

Expected

HCO_3^- = 24 - \Delta AG = 24 - 16 = 8

mEq/L.

Comparing the expected bicarbonate to the actual bicarbonate reveals hidden concurrent metabolic disorders.

4

Compare expected bicarbonate to actual bicarbonate.

The patient's actual

HCO_3^-

is 24 mEq/L, which is 16 mEq/L higher than the expected 8 mEq/L.

Because the actual bicarbonate is significantly higher than what is expected from the ketoacidosis alone, there must be an independent primary process generating bicarbonate. This is a severe Metabolic Alkalosis secondary to the patient's protracted vomiting.

Key Concept

Delta Gap Interpretation in Mixed Acid-Base Disorders

Question 312Question →

Match each classic urinalysis sediment finding with its most likely associated renal pathology.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Muddy brown granular casts match with Acute tubular necrosis. Dysmorphic red blood cells and RBC casts match with Acute glomerulonephritis. White blood cell casts and urinary eosinophils match with Acute interstitial nephritis. Oval fat bodies match with Nephrotic syndrome. Broad, waxy casts match with Advanced chronic kidney disease.

Each urinalysis finding is highly specific to a particular pathophysiologic state in the kidney. Recognizing these classic sediment patterns allows for rapid differentiation between prerenal, intrinsic, and postrenal causes of kidney injury, as well as distinguishing between nephritic and nephrotic syndromes.

Step-by-Step Solution

1

Evaluate the finding of muddy brown granular casts.

Matches with acute tubular necrosis.

These casts represent desquamated, necrotic tubular epithelial cells seen in ischemic or toxic renal injury.

2

Evaluate the finding of dysmorphic red blood cells and RBC casts.

Matches with acute glomerulonephritis.

Glomerular damage allows RBCs to pass into the tubules, where they become deformed (dysmorphic) and trapped in Tamm-Horsfall mucoprotein (casts).

3

Evaluate the finding of WBC casts and urinary eosinophils.

Matches with acute interstitial nephritis.

Interstitial inflammation, often drug-induced, leads to WBC exudation and cast formation in the tubules.

4

Evaluate the finding of oval fat bodies and a 'Maltese cross' appearance.

Matches with nephrotic syndrome.

Severe proteinuria is accompanied by lipiduria; macrophages engulf lipids to form oval fat bodies which exhibit the Maltese cross sign under polarized light.

5

Evaluate the finding of broad, waxy casts.

Matches with advanced chronic kidney disease.

These casts form in dilated tubules that have undergone compensatory hypertrophy due to significant loss of functioning nephrons.

Key Concept

Interpretation of urinalysis sediment is crucial for differentiating the underlying etiology of acute and chronic kidney diseases.

Question 313Question →

During a routine follow-up appointment, a 38-year-old female reports a one-month history of progressive muscle weakness and fatigue. Her past medical history is significant for Sjögren syndrome and multiple episodes of calcium phosphate nephrolithiasis. Vital signs show a blood pressure of 118/76 mm Hg and a heart rate of 84/min. Physical examination reveals dry oral mucosa and 4/5 motor strength in the proximal muscles of the upper and lower extremities.

Laboratory evaluation reveals the following:

Test	Result
Serum sodium	139 mEq/L
Serum potassium	2.9 mEq/L
Serum chloride	115 mEq/L
Serum bicarbonate	14 mEq/L
Blood urea nitrogen	12 mg/dL
Serum creatinine	0.9 mg/dL
Urine sodium	48 mEq/L
Urine potassium	22 mEq/L
Urine chloride	45 mEq/L

Based on these findings, what is this patient's calculated urine anion gap in mEq/L?

Show answer & explanation

Answer: 25

Answer

25

The patient presents with a normal anion gap metabolic acidosis (serum anion gap =

139 - (115 + 14) = 10

mEq/L). To differentiate the etiology of the acidosis, the urine anion gap (UAG) is calculated using the formula

UAG = Urine\ Na^+ + Urine\ K^+ - Urine\ Cl^-

. Substituting the provided urine electrolytes yields

48 + 22 - 45 = 25

mEq/L. A positive UAG indicates that the kidneys are failing to adequately excrete ammonium (

NH_4^+

) to buffer the acid load, pointing toward a renal origin for the acidosis. Specifically, a positive UAG combined with hypokalemia, a history of autoimmune disease (Sjögren syndrome), and recurrent nephrolithiasis is highly characteristic of distal (Type 1) renal tubular acidosis.

Step-by-Step Solution

1

Calculate the serum anion gap to classify the metabolic acidosis.

139 - (115 + 14) = 10

mEq/L

Determining the serum anion gap is the first step in evaluating any metabolic acidosis; a value of 10 mEq/L indicates a non-anion gap (hyperchloremic) metabolic acidosis.

2

Identify the appropriate diagnostic parameter to differentiate the underlying etiology of the non-anion gap metabolic acidosis.

Calculation of the urine anion gap (UAG).

In the setting of a non-anion gap metabolic acidosis, the UAG helps distinguish between renal impaired acid excretion (e.g., renal tubular acidosis) and gastrointestinal bicarbonate loss (e.g., diarrhea).

3

Calculate the urine anion gap using the provided urine electrolytes.

UAG = Urine\ Na^+ + Urine\ K^+ - Urine\ Cl^-

This formula indirectly estimates unmeasured urinary anions, primarily ammonium (

NH_4^+

), which the kidney secretes to clear excess systemic acid.

4

Substitute the patient's urine laboratory values into the formula.

48 + 22 - 45 = 25

mEq/L

A positive value indicates inappropriately low renal ammonium excretion in the face of systemic acidosis, which is consistent with this patient's presentation of distal (type 1) renal tubular acidosis.

Key Concept

The urine anion gap (

UAG = Urine\ Na^+ + Urine\ K^+ - Urine\ Cl^-

) differentiates the etiologies of a normal anion gap metabolic acidosis. A positive UAG implies a renal etiology (e.g., renal tubular acidosis), while a negative UAG points to a gastrointestinal etiology (e.g., severe diarrhea).

Question 314Question →

Match each patient's clinical presentation with the most likely corresponding arterial blood gas and basic metabolic panel profile.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Diabetic ketoacidosis matches with the high anion gap metabolic acidosis profile; COPD exacerbation matches with the acute-on-chronic respiratory acidosis profile; panic attack matches with the acute respiratory alkalosis profile; and vomiting matches with the metabolic alkalosis profile.

Each clinical vignette provides a classic etiology for one of the four primary acid-base disorders. By interpreting the pH to determine the primary disturbance (acidosis vs. alkalosis), and then analyzing the

\text{PaCO}_2

,

\text{HCO}_3^-

, and anion gap, the correct laboratory profile can be definitively paired with the patient's presentation.

Step-by-Step Solution

1

Analyze the first presentation (type 1 diabetes with vomiting and fruity breath).

Identify the primary acid-base disorder as high anion gap metabolic acidosis (HAGMA) secondary to diabetic ketoacidosis.

Ketone body production consumes bicarbonate (

\text{HCO}_3^-

) and increases the anion gap.

2

Match HAGMA to the correct lab profile.

Select the profile with pH 7.25,

\text{HCO}_3^-

14 mEq/L, and an anion gap of 24.

The calculated anion gap (

138 - 100 - 14 = 24

) confirms HAGMA, and the

\text{PaCO}_2

of 30 mm Hg reflects appropriate respiratory compensation per Winter's formula (

1.5 \times 14 + 8 \pm 2 = 29 \pm 2

).

3

Analyze the second presentation (COPD exacerbation).

Identify the primary disorder as respiratory acidosis with chronic partial metabolic compensation.

Airway obstruction causes

\text{CO}_2

retention, while chronic COPD leads to compensatory renal bicarbonate (

\text{HCO}_3^-

) retention.

4

Match respiratory acidosis to the correct lab profile.

Select the profile with pH 7.28,

\text{PaCO}_2

70 mm Hg, and

\text{HCO}_3^-

32 mEq/L.

The elevated

\text{PaCO}_2

directly causes the acidosis, and the elevated bicarbonate indicates chronic renal compensation.

5

Analyze the third presentation (panic attack with hyperventilation).

Identify the primary disorder as acute respiratory alkalosis.

Tachypnea blows off excessive

\text{CO}_2

, rapidly raising the blood pH.

6

Match acute respiratory alkalosis to the correct lab profile.

Select the profile with pH 7.55,

\text{PaCO}_2

24 mm Hg, and

\text{HCO}_3^-

20 mEq/L.

The low

\text{PaCO}_2

drives the alkalemia, and the small drop in bicarbonate represents acute cellular buffering.

7

Analyze the fourth presentation (severe recurrent vomiting).

Identify the primary disorder as metabolic alkalosis.

Loss of gastric hydrochloric acid (

\text{HCl}

) directly leads to alkalosis and hypochloremia.

8

Match metabolic alkalosis to the correct lab profile.

Select the profile with pH 7.52,

\text{PaCO}_2

48 mm Hg,

\text{HCO}_3^-

38 mEq/L, and

\text{Cl}^-

88 mEq/L.

The high bicarbonate defines the alkalosis, hypochloremia (

\text{Cl}^-

88 mEq/L) reflects gastric losses, and the elevated

\text{PaCO}_2

indicates respiratory compensation.

Key Concept

Diagnosis of primary acid-base disorders by analyzing clinical history, pH, PaCO2, bicarbonate, and anion gap.

Question 315Question →

Emergency medical personnel transport a 19-year-old female university student to the emergency department after her roommate found her profoundly lethargic. The roommate reports that the patient had been complaining of severe nausea, subjective fever, and a persistent ringing in her ears for the past 24 hours. Vital signs on arrival include a temperature of 38.3°C (100.9°F), pulse of 118/min, respiratory rate of 30/min, and blood pressure of 105/65 mmHg. Physical examination reveals deep, rapid respirations and dry mucous membranes. A basic metabolic panel and room air arterial blood gas are obtained:

Parameter	Value
Serum sodium	$141$ mEq/L
Serum potassium	$4.1$ mEq/L
Serum chloride	$104$ mEq/L
Serum bicarbonate	$12$ mEq/L
Arterial pH	$7.43$
$PaCO_2$	$18$ mmHg
$PaO_2$	$102$ mmHg

Based on these laboratory results, which of the following best characterizes this patient's acid-base status?

Primary high anion gap metabolic acidosis and primary respiratory alkalosis

Primary high anion gap metabolic acidosis with appropriate respiratory compensation

Primary respiratory alkalosis with appropriate metabolic compensation

Primary normal anion gap metabolic acidosis and primary respiratory alkalosis

Primary high anion gap metabolic acidosis, primary respiratory alkalosis, and primary metabolic alkalosis

Show answer & explanation

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

Answer

The patient's laboratory values reveal a primary high anion gap metabolic acidosis and a concurrent primary respiratory alkalosis.

The correct answer identifies that this patient has two distinct primary acid-base disorders. The patient's presentation of tinnitus, tachypnea, nausea, and fever is the classic triad of salicylate (aspirin) toxicity. Salicylates directly stimulate the medullary respiratory center, causing a primary respiratory alkalosis (manifested by the inappropriately low

PaCO_2

of

18

mmHg). Simultaneously, salicylates uncouple oxidative phosphorylation, leading to the accumulation of organic acids (lactic acid, ketoacids) and establishing a primary high anion gap metabolic acidosis (anion gap of

25

mEq/L). The opposing effects of these two primary disorders result in a near-normal, slightly alkalemic pH of

7.43

.

Step-by-Step Solution

1

Examine the arterial pH to determine the overall acid-base status.

The pH is

7.43

, which leans toward alkalemia but is within the normal range (

7.35 - 7.45

). This strongly suggests a mixed disorder given the significantly abnormal bicarbonate and

PaCO_2

.

Normal pH in the setting of abnormal

HCO_3^-

and

PaCO_2

almost always indicates a mixed acid-base disorder.

2

Calculate the serum anion gap (

Na^+ - [Cl^- + HCO_3^-]

).

Anion Gap =

141 - (104 + 12) = 141 - 116 = 25

mEq/L.

An elevated anion gap (normal is

8-12

mEq/L) diagnoses a primary high anion gap metabolic acidosis (HAGMA), regardless of the serum pH.

3

Apply Winter's formula (

Expected PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2

) to assess respiratory compensation.

Expected PaCO_2 = 1.5 \times 12 + 8 \pm 2 = 18 + 8 \pm 2 = 26 \pm 2

mmHg. The expected range is

24-28

mmHg.

Winter's formula determines if the lungs are appropriately compensating for the metabolic acidosis.

4

Compare actual

PaCO_2

to expected

PaCO_2

.

The actual

PaCO_2

(

18

mmHg) is significantly lower than the expected range (

24-28

mmHg).

When actual

PaCO_2

is less than the expected compensatory

PaCO_2

, it indicates a concurrent primary respiratory alkalosis.

5

Calculate the delta-delta gap (

\Delta AG / \Delta HCO_3^-

) to check for a third disorder.

\Delta AG = 25 - 12 = 13

.

\Delta HCO_3^- = 24 - 12 = 12

. Ratio is

13/12 \approx 1.1

.

A ratio between 1 and 2 indicates an uncomplicated HAGMA without an underlying NAGMA or metabolic alkalosis.

Key Concept

Mixed acid-base disorders are diagnosed through a stepwise approach: determining primary disturbance, calculating the anion gap, assessing appropriate respiratory compensation with Winter's formula, and checking the delta-delta ratio.

Question 316Question →

An internal medicine resident is evaluating a patient recently admitted to the progressive care unit. The patient has a long-standing history of restrictive eating patterns and admits to consuming large quantities of over-the-counter stimulant laxatives over the past month to lose weight. Today, she developed profound, diffuse muscle weakness to the point of being unable to lift her arms from her sides, prompting her roommate to call emergency services.

Current vital signs include a blood pressure of 92/58 mm Hg, heart rate of 112/min, and respiratory rate of 10/min with shallow excursions.

Laboratory evaluation shows the following:

Laboratory Test	Result	Reference Range
Serum sodium	139 mEq/L	136-145 mEq/L
Serum potassium	1.9 mEq/L	3.5-5.0 mEq/L
Serum chloride	116 mEq/L	98-106 mEq/L
Serum bicarbonate	14 mEq/L	22-28 mEq/L
Blood urea nitrogen	34 mg/dL	7-18 mg/dL
Serum creatinine	1.3 mg/dL	0.6-1.2 mg/dL

Arterial blood gas analysis on room air:

Parameter	Result	Reference Range
pH	7.21	7.35-7.45
$pCO_2$	36 mm Hg	35-45 mm Hg
$pO_2$	85 mm Hg	75-105 mm Hg

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

Normal anion gap metabolic acidosis and concomitant respiratory acidosis

Normal anion gap metabolic acidosis with appropriate respiratory compensation

Normal anion gap metabolic acidosis and concomitant respiratory alkalosis

High anion gap metabolic acidosis with appropriate respiratory compensation

High anion gap metabolic acidosis and concomitant respiratory acidosis

Show answer & explanation

Answer: Normal anion gap metabolic acidosis and concomitant respiratory acidosis

Answer

The patient has a normal anion gap metabolic acidosis (due to laxative-induced bicarbonate loss) accompanied by a superimposed respiratory acidosis (due to inadequate respiratory compensation, likely secondary to profound hypokalemia-induced diaphragmatic weakness).

The correct answer correctly identifies two distinct processes. First, there is a normal anion gap metabolic acidosis (NAGMA). The anion gap is

139 - (116 + 14) = 9\ mEq/L

. NAGMA is expected here due to significant gastrointestinal bicarbonate loss from laxative abuse. Second, there is a concomitant respiratory acidosis. According to Winter's formula (

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

), the expected

pCO_2

is 27 to 31 mm Hg. The patient's actual

pCO_2

is 36 mm Hg. Because the actual

pCO_2

is higher than the maximum expected value, a superimposed respiratory acidosis is present. Clinically, this is explained by her profound hypokalemia (

1.9\ mEq/L

), which causes severe skeletal muscle weakness that extends to the diaphragm, resulting in hypoventilation.

Step-by-Step Solution

1

Determine the primary acid-base disturbance.

The patient has acidemia (pH 7.21) with a low bicarbonate (14 mEq/L), confirming a primary metabolic acidosis.

Identifying the primary disorder directs the subsequent calculation of the anion gap and expected respiratory compensation.

2

Calculate the serum anion gap.

Anion\ Gap = Na^+ - (Cl^- + HCO_3^-) = 139 - (116 + 14) = 9\ mEq/L

. This is a normal anion gap.

Differentiates between normal anion gap (hyperchloremic) and high anion gap etiologies. Laxative abuse causes severe diarrhea, leading to direct gastrointestinal loss of bicarbonate.

3

Calculate the expected respiratory compensation using Winter's formula.

Expected\ pCO_2 = (1.5 \times 14) + 8 \pm 2 = 21 + 8 \pm 2 = 29 \pm 2

. The expected

pCO_2

range is 27 to 31 mm Hg.

Determines if the patient's respiratory drive is appropriately compensating for the metabolic acidosis.

4

Compare actual

pCO_2

to expected

pCO_2

.

The actual

pCO_2

(36 mm Hg) is strictly greater than the expected range (27-31 mm Hg).

An actual

pCO_2

higher than expected defines a concomitant (superimposed) respiratory acidosis. In this clinical scenario, severe hypokalemia (1.9 mEq/L) is causing profound muscle weakness, including diaphragmatic impairment, leading to relative hypoventilation.

Key Concept

Patients with severe hypokalemia can develop respiratory muscle weakness leading to hypoventilation, which presents on an arterial blood gas as a superimposed respiratory acidosis (actual

pCO_2

> expected

pCO_2

by Winter's formula).

Question 317Question →

A 48-year-old man is brought to the emergency department by emergency medical services after his neighbor found him unresponsive in his garage. An open, empty container of automotive antifreeze was found nearby. On examination, he is deeply lethargic and responds only to painful stimuli. His vital signs show a blood pressure of $110/70 \text{ mmHg}$ , heart rate of $105/\text{min}$ , and respiratory rate of $24/\text{min}$ .

Laboratory testing reveals the following:

Parameter	Value
Serum sodium	$142 \text{ mEq/L}$
Serum potassium	$4.5 \text{ mEq/L}$
Serum chloride	$104 \text{ mEq/L}$
Serum bicarbonate	$10 \text{ mEq/L}$
Blood urea nitrogen	$28 \text{ mg/dL}$
Serum glucose	$126 \text{ mg/dL}$
Measured serum osmolality	$345 \text{ mOsm/kg H}_2\text{O}$
Blood ethanol	Undetectable

Based on the laboratory findings, what is the patient's calculated serum osmolar gap?

Show answer & explanation

Answer: 44

Answer

44

The patient's osmolar gap is calculated by subtracting the calculated serum osmolality from the measured serum osmolality. Calculated osmolality is found using the formula:

2 \times Na + \frac{BUN}{2.8} + \frac{Glucose}{18}

. Plugging in the values yields:

2 \times 142 + \frac{28}{2.8} + \frac{126}{18} = 284 + 10 + 7 = 301 \text{ mOsm/kg H}_2\text{O}

. The measured osmolality is

345 \text{ mOsm/kg H}_2\text{O}

. The osmolar gap is therefore

345 - 301 = 44

. A normal osmolar gap is

\leq 10

. In a patient with an unexplained high anion gap metabolic acidosis and altered mental status, an elevated osmolar gap strongly suggests toxic alcohol ingestion, such as ethylene glycol (which is found in antifreeze).

Step-by-Step Solution

1

Recall the formula for calculated serum osmolality.

Calculated Osmolality =

2 \times Na + \frac{BUN}{2.8} + \frac{Glucose}{18}

This formula estimates the osmolal contribution of the major endogenous serum solutes (sodium, urea, and glucose).

2

Substitute the provided laboratory values into the formula.

2 \times 142 + \frac{28}{2.8} + \frac{126}{18}

Accurate substitution of sodium, BUN, and glucose is required to find the expected baseline osmolality.

3

Perform the mathematical calculation.

284 + 10 + 7 = 301 \text{ mOsm/kg H}_2\text{O}

These values represent the standard milliosmoles per kilogram of water contributed by normal solutes.

4

Subtract the calculated osmolality from the measured serum osmolality to find the osmolar gap.

345 - 301 = 44

An elevated osmolar gap (>10) suggests the presence of unmeasured osmotically active substances, such as toxic alcohols (e.g., ethylene glycol).

Key Concept

The serum osmolar gap is used to detect the presence of unmeasured, osmotically active agents (like ethylene glycol or methanol) and is found by subtracting calculated osmolality from measured osmolality.

Question 318Question →

A resident is evaluating four patients who all present with significant hypokalemia and metabolic alkalosis. Pair each patient's specific clinical and laboratory profile with the most likely underlying etiology.

Click a left item, then click its matching right item

Show answer & explanation

Answer

The normal-blood-pressure patient with low urine chloride matches surreptitious vomiting; the normal-blood-pressure patient with high urine chloride matches diuretic abuse; the hypertensive patient with low renin matches primary hyperaldosteronism; and the hypertensive patient with high renin matches renovascular hypertension.

Evaluating hypokalemic metabolic alkalosis requires a structured approach. First, assess blood pressure. If normal, use urine chloride to distinguish GI losses (low Cl) from renal losses (high Cl). If hypertensive, use the aldosterone-to-renin ratio to differentiate primary (low renin) from secondary (high renin) hyperaldosteronism.

Step-by-Step Solution

1

Categorize the patients based on their blood pressure status.

The two younger female patients are normotensive, whereas the two older male patients are hypertensive.

Blood pressure distinguishes normotensive volume-depleted states from hypertensive mineralocorticoid excess states.

2

Differentiate the two normotensive patients using spot urine chloride concentrations.

The patient with a low urine chloride (< 20 mEq/L) is experiencing gastrointestinal losses (vomiting), while the patient with a high urine chloride (> 20 mEq/L) is experiencing renal losses (diuretic abuse).

The kidney's ability to conserve chloride indicates whether the primary defect is extrarenal (allowing appropriate renal compensation) or intrarenal (tubular blockade).

3

Differentiate the two hypertensive patients using the aldosterone and renin profiles.

The patient with low renin has primary adrenal disease, while the patient with high renin has secondary hyperaldosteronism from reduced renal blood flow.

Unregulated adrenal secretion directly suppresses renin via negative feedback, whereas impaired renal perfusion actively stimulates renin release.

Key Concept

Algorithm for evaluating hypokalemic metabolic alkalosis using blood pressure, urine chloride, and renin-aldosterone levels.

Question 319Question →

A 45-year-old man is evaluated in the emergency department for a 3-day history of intractable nausea, nonbilious vomiting, and diffuse abdominal pain. He has a known history of severe alcohol use disorder and reports his last alcoholic beverage was approximately 48 hours ago. On physical examination, he appears agitated, diaphoretic, and tremulous.

Vital signs are:
Temperature: $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ )
Blood pressure: $105/65\text{ mm Hg}$
Heart rate: $115/\text{min}$
Respiratory rate: $26/\text{min}$

Laboratory studies show:

Test	Result	Reference Range
Sodium	$140\text{ mEq/L}$	$136-145\text{ mEq/L}$
Potassium	$3.1\text{ mEq/L}$	$3.5-5.0\text{ mEq/L}$
Chloride	$90\text{ mEq/L}$	$98-106\text{ mEq/L}$
Bicarbonate	$16\text{ mEq/L}$	$22-28\text{ mEq/L}$
Blood urea nitrogen	$38\text{ mg/dL}$	$8-20\text{ mg/dL}$
Creatinine	$1.7\text{ mg/dL}$	$0.7-1.3\text{ mg/dL}$
Glucose	$85\text{ mg/dL}$	$70-110\text{ mg/dL}$

Arterial blood gas analysis on room air reveals:

Test	Result	Reference Range
pH	$7.43$	$7.35-7.45$
$\text{pCO}_2$	$25\text{ mm Hg}$	$35-45\text{ mm Hg}$
$\text{pO}_2$	$92\text{ mm Hg}$	$75-105\text{ mm Hg}$

Urinalysis is strongly positive for ketones.

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

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

High anion gap metabolic acidosis and respiratory alkalosis

High anion gap metabolic acidosis and metabolic alkalosis with appropriate respiratory compensation

Primary respiratory alkalosis with compensatory metabolic acidosis

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

Show answer & explanation

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

Answer

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

This patient's clinical presentation points to three distinct acid-base insults: alcoholic ketoacidosis, vomiting-induced volume depletion, and alcohol withdrawal-mediated hyperventilation. The diagnosis of a triple acid-base disorder is mathematically confirmed in three steps. First, the Anion Gap is

140 - (90 + 16) = 34\text{ mEq/L}

, confirming a High Anion Gap Metabolic Acidosis. Second, Winter's formula dictates the expected

\text{pCO}_2

should be

(1.5 \times 16) + 8 = 32\text{ mm Hg}

. The actual

\text{pCO}_2

is

25\text{ mm Hg}

, indicating a primary Respiratory Alkalosis is driving the

\text{CO}_2

down further than compensation alone. Third, the Delta Gap (

\Delta\text{AG}

) is

34 - 12 = 22

. Adding this back to the measured bicarbonate yields a corrected bicarbonate of

16 + 22 = 38\text{ mEq/L}

. Because this is significantly greater than

24\text{ mEq/L}

, a concurrent Metabolic Alkalosis is confirmed.

Step-by-Step Solution

1

Calculate the Anion Gap (AG) to identify the first disorder.

\text{AG} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (90 + 16) = 34\text{ mEq/L}

.

Since the normal AG is roughly 12, a value of 34 confirms a High Anion Gap Metabolic Acidosis (HAGMA), likely alcoholic ketoacidosis given the history and positive ketones.

2

Apply Winter's formula to determine if respiratory compensation is appropriate.

\text{Expected pCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2 = (1.5 \times 16) + 8 \pm 2 = 32 \pm 2

(range

30-34\text{ mm Hg}

). The measured

\text{pCO}_2

is

25\text{ mm Hg}

.

Because the measured

\text{pCO}_2

(25) is lower than the expected compensatory range (30-34), the patient is hyperventilating more than expected, confirming a concurrent primary Respiratory Alkalosis (likely from alcohol withdrawal and agitation).

3

Calculate the Delta-Delta Gap (

\Delta/\Delta

) to uncover hidden metabolic disorders.

\Delta\text{AG} = \text{Calculated AG} - \text{Normal AG} = 34 - 12 = 22

.

\text{Corrected HCO}_3^- = \text{Measured HCO}_3^- + \Delta\text{AG} = 16 + 22 = 38\text{ mEq/L}

.

The corrected

\text{HCO}_3^-

estimates what the bicarbonate would be if the HAGMA were not present. Since

38\text{ mEq/L}

is much higher than a normal baseline of

24\text{ mEq/L}

, a concurrent primary Metabolic Alkalosis is also present, secondary to the patient's severe vomiting.

Key Concept

The systematic diagnosis of mixed acid-base disorders requires calculating the anion gap to identify HAGMA, applying Winter's formula to assess respiratory compensation, and utilizing the delta-delta gap to unmask hidden concurrent metabolic disorders.

Question 320Question →

A $28$ -year-old woman with a history of type 1 diabetes mellitus presents to the hospital after $4$ days of worsening abdominal pain, severe nausea, and frequent episodes of nonbilious vomiting. She has missed several doses of her basal insulin due to her inability to tolerate oral intake. On arrival, her blood pressure is $98/62 \text{ mm Hg}$ , and her pulse is $115/\text{min}$ . Physical examination reveals significantly dry mucous membranes, decreased skin turgor, and diffuse mild abdominal tenderness without rebound or guarding.

Laboratory values are as follows:

Serum sodium: $138 \text{ mEq/L}$
Serum potassium: $4.8 \text{ mEq/L}$
Serum chloride: $92 \text{ mEq/L}$
Serum bicarbonate: $14 \text{ mEq/L}$
Arterial pH: $7.28$
Arterial $PaCO_2$ : $29 \text{ mm Hg}$

Assume a normal baseline anion gap of $12 \text{ mEq/L}$ .

What is the patient's calculated corrected bicarbonate level in $\text{mEq/L}$ ?

Show answer & explanation

Answer: 34

Answer

The calculated corrected bicarbonate level is 34 mEq/L.

The correct calculation yields

34 \text{ mEq/L}

. A corrected bicarbonate value significantly higher than the normal range (

>24-26 \text{ mEq/L}

) indicates that a concurrent primary metabolic alkalosis is present. In this clinical scenario, the metabolic alkalosis is driven by the patient's intractable vomiting, which causes volume depletion and loss of hydrochloric acid, masking the absolute severity of the bicarbonate deficit caused by the diabetic ketoacidosis.

Step-by-Step Solution

1

Calculate the Anion Gap (AG).

32 \text{ mEq/L}

An elevated anion gap confirms the presence of a primary high anion gap metabolic acidosis (HAGMA), which is typical in diabetic ketoacidosis.

2

Calculate the Delta Gap (

\Delta AG

).

20 \text{ mEq/L}

The delta gap represents the amount of unmeasured acid anions that have accumulated and consumed bicarbonate.

3

Calculate the corrected bicarbonate.

34 \text{ mEq/L}

Adding the delta gap back to the measured bicarbonate reveals what the bicarbonate level would be if the HAGMA were not present, helping uncover additional acid-base disorders.

Key Concept

Delta-delta calculation (corrected bicarbonate) to identify mixed acid-base disorders

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