Nephrology Practice Questions — USMLE Step 2 CK — Page 9

Question 161Question →

During bedside rounds in the surgical intensive care unit, a critical care team evaluates a 68-year-old individual who underwent an emergency exploratory laparotomy 24 hours ago for acute mesenteric ischemia. The postoperative course has been complicated by massive fluid shifts and a need for continuous nasogastric tube suctioning, which is currently draining approximately 1,500 mL of bilious fluid daily. The patient is intubated and sedated. Vital signs demonstrate a blood pressure of 88/50 mm Hg and a heart rate of 122/min.

Serum laboratory analysis reveals:
$Na^+$ : 142 mEq/L
$K^+$ : 3.2 mEq/L
$Cl^-$ : 88 mEq/L
$HCO_3^-$ : 16 mEq/L
BUN: 45 mg/dL
Creatinine: 2.1 mg/dL

Arterial blood gas analysis on 40% $FiO_2$ demonstrates:
pH: 7.28
$P_{CO2}$ : 34 mm Hg
$P_{O2}$ : 92 mm Hg

Assuming a reference normal anion gap of 12 mEq/L and a reference normal bicarbonate of 24 mEq/L, calculate the patient's corrected bicarbonate level (often used to assess for a tertiary acid-base disorder via the delta-delta gap) in mEq/L.

Show answer & explanation

Answer: 42

Answer

The patient's corrected bicarbonate is 42 mEq/L.

The correct value is 42 mEq/L. The patient has a high anion gap metabolic acidosis (AG = 38) driven by lactic acidosis from mesenteric ischemia. However, the patient is also undergoing continuous nasogastric suctioning, which removes

H^+

and

Cl^-

, creating a concurrent metabolic alkalosis. By calculating the

\Delta AG

(

38 - 12 = 26

) and adding it back to the measured

HCO_3^-

(16), we find the corrected bicarbonate is 42 mEq/L. Because 42 mEq/L is significantly higher than the normal 24 mEq/L, it confirms the presence of a severe concomitant metabolic alkalosis.

Step-by-Step Solution

1

Calculate the serum anion gap.

142 - (88 + 16) = 38

mEq/L

Establishing the presence and magnitude of a primary high anion gap metabolic acidosis (HAGMA) due to mesenteric ischemia (lactic acidosis).

2

Calculate the excess anion gap (

\Delta AG

).

38 - 12 = 26

mEq/L

Determining how much unmeasured anion has accumulated, which represents exactly how much bicarbonate was consumed to buffer it.

3

Add the

\Delta AG

back to the measured serum bicarbonate.

16 + 26 = 42

mEq/L

This yields the corrected bicarbonate (or starting bicarbonate) before the HAGMA occurred, revealing any hidden concurrent metabolic disorders.

Key Concept

Delta-delta calculation for diagnosing mixed acid-base disorders

Question 162Question →

A newly admitted 19-year-old male with a history of type 1 diabetes mellitus is evaluated on the medical ward for persistent nausea, recurrent vomiting, and diffuse abdominal pain. The patient admits to missing his insulin doses over the past three days. The following initial serum chemistry panel is obtained:

* Sodium: 138 mEq/L
* Potassium: 5.1 mEq/L
* Chloride: 99 mEq/L
* Bicarbonate: 14 mEq/L
* Blood urea nitrogen: 28 mg/dL
* Creatinine: 1.2 mg/dL
* Glucose: 485 mg/dL

Calculate the patient's serum anion gap in mEq/L.

Show answer & explanation

Answer: 25

Answer

25 mEq/L

The patient's clinical presentation and laboratory results indicate diabetic ketoacidosis. To evaluate the severity and nature of the metabolic acidosis, the anion gap is calculated using the formula

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

. Using the provided serum values:

138 - (99 + 14) = 25

mEq/L. A normal anion gap typically ranges between 8 and 12 mEq/L, confirming that this patient has a significantly elevated high anion gap metabolic acidosis (HAGMA).

Step-by-Step Solution

1

Identify the standard formula for calculating the serum anion gap.

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

The anion gap evaluates the unmeasured anions in the blood by subtracting the sum of the primary measured anions (chloride and bicarbonate) from the primary measured cation (sodium).

2

Extract the necessary laboratory values from the clinical vignette.

Na^+ = 138

mEq/L,

Cl^- = 99

mEq/L, and

HCO_3^- = 14

mEq/L.

These specific serum electrolytes are required to perform the calculation.

3

Perform the subtraction using the extracted values.

138 - (99 + 14) = 25

mEq/L.

The result yields the final unmeasured anion concentration, which is significantly elevated and consistent with a high anion gap metabolic acidosis.

Key Concept

Calculation of the serum anion gap in the evaluation of metabolic acidosis.

Question 163Question →

Match each clinical and laboratory profile of hypokalemic metabolic alkalosis to its most likely underlying etiology.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Primary hyperaldosteronism is characterized by hypertension and high aldosterone; Surreptitious vomiting presents with normotension and low urine chloride; Gitelman syndrome presents with normotension, high urine chloride, and hypocalciuria; Liddle syndrome presents with hypertension and low aldosterone; and Bartter syndrome presents with normotension, high urine chloride, and hypercalciuria.

The diagnostic algorithm for metabolic alkalosis requires synthesis of multiple variables. Volume-expanded states present with hypertension, and the aldosterone level distinguishes primary hyperaldosteronism (high) from Liddle syndrome (low). Volume-contracted (normotensive) states are distinguished by urine chloride: low urine chloride indicates extrarenal loss like vomiting, while high urine chloride indicates a renal tubular defect. Finally, tubular defects are differentiated by calcium excretion (hypercalciuria in Bartter vs. hypocalciuria in Gitelman).

Step-by-Step Solution

1

Evaluate blood pressure to categorize the etiologies of hypokalemic metabolic alkalosis into hypertensive and normotensive states.

Hypertensive conditions include Primary hyperaldosteronism and Liddle syndrome; normotensive conditions include Surreptitious vomiting, Gitelman syndrome, and Bartter syndrome.

Blood pressure serves as a surrogate for effective circulating volume, immediately dividing the differential diagnosis into volume-expanded versus volume-contracted etiologies.

2

For hypertensive profiles, assess the plasma renin activity and serum aldosterone levels.

Elevated aldosterone with suppressed renin points to Primary hyperaldosteronism. Suppressed aldosterone with suppressed renin points to Liddle syndrome.

Liddle syndrome is caused by an autonomous ENaC channel activation, which expands volume and suppresses both renin and aldosterone, distinguishing it from an aldosterone-producing adenoma.

3

For normotensive profiles, analyze the urine chloride concentration to determine saline responsiveness.

A urine chloride < 10 mEq/L indicates a saline-responsive state (e.g., vomiting). A urine chloride > 20 mEq/L indicates a saline-resistant tubular disorder.

In extrarenal chloride loss (vomiting), the kidneys appropriately conserve chloride. In intrinsic renal tubular wasting, the kidneys inappropriately excrete chloride despite volume depletion.

4

Differentiate the saline-resistant normotensive profiles using urine calcium and serum magnesium levels.

Hypocalciuria and hypomagnesemia identify Gitelman syndrome. Hypercalciuria identifies Bartter syndrome.

Gitelman syndrome shares the physiology of thiazide diuretics (DCT defect), while Bartter syndrome shares the physiology of loop diuretics (thick ascending limb defect).

Key Concept

Evaluation of metabolic alkalosis relies on sequentially analyzing blood pressure, urine chloride concentration, and the renin-aldosterone axis.

Question 164Question →

A 28-year-old daycare worker comes to the urgent care center complaining of severe, watery diarrhea for the past four days. She reports drinking only tap water since her symptoms began and has been unable to keep solid food down. On physical examination, her blood pressure is 105/65 mmHg and her pulse is 112/min. Her mucous membranes are dry. Laboratory studies show:

Lab	Value
Sodium	138 mEq/L
Potassium	3.2 mEq/L
Chloride	113 mEq/L
Bicarbonate	14 mEq/L
Blood urea nitrogen	35 mg/dL
Creatinine	1.1 mg/dL

Which of the following is the expected arterial partial pressure of carbon dioxide ( $PaCO_2$ ) for this patient if appropriate respiratory compensation has occurred?

14 mmHg

21 mmHg

29 mmHg

40 mmHg

48 mmHg

Show answer & explanation

Answer: 29 mmHg

Answer

29 mmHg

The patient has a normal anion gap metabolic acidosis secondary to severe diarrhea (loss of bicarbonate in the stool). To determine if her lungs are appropriately compensating by blowing off carbon dioxide, Winter's formula must be applied. The formula is: Expected

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

. Given a bicarbonate of 14 mEq/L, the calculation is

1.5 \times 14 + 8 = 21 + 8 = 29

. Therefore, the expected

PaCO_2

is

29 \pm 2

mmHg. A measured

PaCO_2

exactly in this range indicates appropriate respiratory compensation.

Step-by-Step Solution

1

Calculate the anion gap to characterize the primary acid-base disturbance.

Anion Gap =

138 - (113 + 14) = 11

mEq/L.

An anion gap between 8 and 12 mEq/L indicates a normal anion gap metabolic acidosis (NAGMA), consistent with severe gastrointestinal bicarbonate loss from diarrhea.

2

Calculate the expected respiratory compensation using Winter's formula.

Expected

PaCO_2 = (1.5 \times 14) + 8 = 21 + 8 = 29

mmHg.

Winter's formula (

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

) is used to determine if the respiratory response to a metabolic acidosis is appropriate, inadequate, or excessive.

Key Concept

In patients with metabolic acidosis, the expected compensatory respiratory response (decrease in

PaCO_2

) is calculated using Winter's formula: Expected

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

.

Question 165Question →

Match each classic urinalysis sediment finding with the renal pathology it most strongly indicates.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Muddy brown casts match with acute tubular necrosis; RBC casts with acute glomerulonephritis; WBC casts with acute pyelonephritis; and fatty casts with nephrotic syndrome.

Each specific cellular cast is highly specific to the pathophysiology of the underlying renal disease. RBCs passing through a damaged glomerulus form RBC casts. WBCs entering tubules during infection form WBC casts. Sloughed necrotic cells form muddy brown casts. Heavy lipiduria leads to fatty casts.

Step-by-Step Solution

1

Evaluate the clinical significance of muddy brown granular casts.

Matches with acute tubular necrosis.

These casts are formed by necrotic tubular epithelial cells that slough off into the tubular lumen during ischemic or toxic insults to the kidneys.

2

Evaluate the clinical significance of red blood cell (RBC) casts.

Matches with acute glomerulonephritis.

RBC casts form when red blood cells traverse a damaged glomerular basement membrane and aggregate in the tubules, indicating nephritic conditions.

3

Evaluate the clinical significance of white blood cell (WBC) casts.

Matches with acute pyelonephritis.

WBC casts indicate inflammation in the renal interstitium and tubules, strongly pointing towards pyelonephritis rather than a simple lower urinary tract infection.

4

Evaluate the clinical significance of fatty casts and oval fat bodies.

Matches with nephrotic syndrome.

Severe proteinuria in nephrotic syndrome is accompanied by lipiduria. Macrophages and tubular cells engulf the lipids, forming oval fat bodies.

Key Concept

Specific cellular and acellular casts in urinalysis sediment provide essential diagnostic clues for localizing the mechanism and site of acute or chronic renal injury.

Question 166Question →

A 21-year-old male is brought to the emergency department by his roommate due to profound generalized weakness and episodes of confusion over the past 24 hours. The roommate mentions the patient was recently fired from his job at a paint manufacturing plant and has been spending most of his time locked in his room with a strong, unusual chemical odor. On physical examination, the patient is lethargic but arousable. Muscle strength is 2/5 in the bilateral lower extremities and 3/5 in the upper extremities. Deep tendon reflexes are diminished. His blood pressure is 110/70 mm Hg, heart rate is 98/min, and respiratory rate is 22/min.

Laboratory studies reveal the following:

Parameter	Value
Sodium	$137 \text{ mEq/L}$
Potassium	$2.1 \text{ mEq/L}$
Chloride	$115 \text{ mEq/L}$
Bicarbonate	$12 \text{ mEq/L}$
Blood urea nitrogen	$12 \text{ mg/dL}$
Creatinine	$0.8 \text{ mg/dL}$

Arterial blood gas on room air shows a pH of 7.28 and a $pCO_2$ of 26 mm Hg. Urinalysis reveals a pH of 6.8 with no protein or blood. Urine electrolyte testing reveals a sodium of 45 mEq/L, potassium of 25 mEq/L, and chloride of 30 mEq/L.

Which of the following is the most likely primary mechanism responsible for this patient's laboratory abnormalities?

Impaired distal tubular secretion of hydrogen ions

Decreased proximal tubular reabsorption of filtered bicarbonate

Reduced aldosterone synthesis or tubular responsiveness

Accumulation of unmeasured endogenous or exogenous organic acids

Excessive gastrointestinal loss of bicarbonate-rich fluids

Show answer & explanation

Answer: Impaired distal tubular secretion of hydrogen ions

Answer

Impaired distal tubular secretion of hydrogen ions

The patient's presentation is highly characteristic of inhalant abuse, specifically toluene sniffing. Toluene is metabolized in the liver to hippuric acid. The rapid renal excretion of the hippurate anion, accompanied by sodium and potassium to maintain electrical neutrality, results in profound hypokalemia and severe muscle weakness. Because the hippurate anion is excreted rather than accumulating in the serum, the patient develops a hyperchloremic (normal anion gap) metabolic acidosis. Furthermore, toluene induces a functional Type 1 (distal) renal tubular acidosis, which is characterized by the inability of the distal tubule to secrete hydrogen ions. This failure of distal acidification is confirmed by the inappropriately high urine pH (6.8) in the face of systemic acidemia and a markedly positive urine anion gap (+40 mEq/L), reflecting deficient urinary ammonium (

NH_4^+

) excretion.

Step-by-Step Solution

1

Calculate the serum anion gap.

Na^+ - (Cl^- + HCO_3^-) = 137 - (115 + 12) = 10 \text{ mEq/L}

.

This establishes that the patient has a normal anion gap metabolic acidosis (NAGMA), ruling out high-gap toxic alcohol ingestions.

2

Assess respiratory compensation using Winter's formula.

Expected

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

. The measured

pCO_2

is 26 mm Hg.

This confirms there is no superimposed primary respiratory acid-base disorder.

3

Calculate the urine anion gap (UAG) to determine the etiology of the NAGMA.

U_{Na} + U_{K} - U_{Cl} = 45 + 25 - 30 = +40 \text{ mEq/L}

.

A positive UAG indicates impaired renal

NH_4^+

excretion, confirming a renal etiology (Renal Tubular Acidosis) rather than gastrointestinal bicarbonate loss.

4

Synthesize the laboratory data with the clinical presentation.

The patient has a positive UAG, severe hypokalemia, and a urine pH > 5.5, which defines Type 1 (distal) RTA. The history of inhalant abuse points to toluene toxicity.

Toluene is metabolized to hippuric acid. The protons consume bicarbonate (causing acidosis), while the hippurate anion is rapidly excreted in the urine with sodium and potassium, causing profound hypokalemia, volume depletion, and a normal serum anion gap.

Key Concept

Toluene toxicity causes a Type 1 (distal) renal tubular acidosis, characterized by a normal serum anion gap metabolic acidosis, severe hypokalemia, inappropriately alkaline urine pH, and a positive urine anion gap.

Question 167Question →

A 42-year-old woman presents to the emergency department with a 2-day history of profound, generalized muscle weakness. She has a past medical history of rheumatoid arthritis and complains of chronic dry eyes and a dry mouth. She takes no daily medications. Her blood pressure is 118/76 mm Hg, heart rate is 88/min, and respiratory rate is 18/min. Physical examination reveals normal muscle bulk but symmetrically decreased proximal and distal muscle strength (3/5) in both the upper and lower extremities. Deep tendon reflexes are globally diminished. Laboratory evaluation reveals the following:

Serum chemistry:
Sodium: 139 mEq/L
Potassium: 2.1 mEq/L
Chloride: 114 mEq/L
Bicarbonate: 14 mEq/L
Blood urea nitrogen: 14 mg/dL
Creatinine: 0.9 mg/dL

Arterial blood gas (room air):
pH: 7.28
pCO2: 30 mm Hg
pO2: 95 mm Hg

Urinalysis:
pH: 6.8
Specific gravity: 1.015
Protein: Negative
Glucose: Negative
Ketones: Negative

Which of the following is the primary pathophysiologic mechanism underlying this patient's laboratory abnormalities?

Impaired hydrogen ion secretion in the alpha-intercalated cells

Decreased bicarbonate reabsorption in the proximal convoluted tubule

Reduced responsiveness to aldosterone at the principal cells

Accumulation of unmeasured endogenous organic acids in the serum

Severe gastrointestinal loss of bicarbonate-rich fluids

Show answer & explanation

Answer: Impaired hydrogen ion secretion in the alpha-intercalated cells

Answer

Impaired hydrogen ion secretion in the alpha-intercalated cells

This patient presents with a severe primary metabolic acidosis. The first step is to calculate the anion gap:

Na - (Cl + HCO_3) = 139 - (114 + 14) = 11

mEq/L. Because this is within the normal range (8-12 mEq/L), she has a normal anion gap metabolic acidosis (NAGMA). To narrow down the etiology of a NAGMA, one must evaluate the serum potassium and urine pH. The normal renal response to systemic acidemia is to maximize acid excretion, which should lower the urine pH below 5.5. This patient has an inappropriately alkaline urine (pH 6.8) coupled with severe hypokalemia (2.1 mEq/L). This presentation is classic for Type 1 (distal) renal tubular acidosis (RTA).

Type 1 RTA is characterized by a defect in the alpha-intercalated cells of the distal tubule and collecting duct, leading to an inability to secrete hydrogen ions into the tubular lumen. Because hydrogen cannot be excreted, potassium is excreted in its place to maintain electrical neutrality, leading to profound hypokalemia. The severe hypokalemia explains this patient's muscle weakness and diminished reflexes. Type 1 RTA is strongly associated with autoimmune disorders, particularly Sjögren syndrome, which is heavily suggested by this patient's chronic dry eyes (xerophthalmia), dry mouth (xerostomia), and history of rheumatoid arthritis.

Step-by-Step Solution

1

Calculate the serum anion gap to classify the metabolic acidosis.

The anion gap is calculated as

Na - (Cl + HCO_3) = 139 - (114 + 14) = 11

mEq/L.

An anion gap of 11 mEq/L is within the normal range (8-12 mEq/L), indicating a normal anion gap metabolic acidosis (NAGMA) rather than a high anion gap metabolic acidosis.

2

Calculate the expected pCO2 using Winter's formula to ensure appropriate respiratory compensation.

Expected

pCO_2 = 1.5(HCO_3) + 8 = 1.5(14) + 8 = 29 \pm 2

mm Hg. The actual pCO2 is 30 mm Hg.

The measured pCO2 matches the expected value, confirming an appropriately compensated primary metabolic acidosis without a superimposed respiratory disorder.

3

Evaluate the serum potassium and urine pH to differentiate the causes of NAGMA.

The patient has severe hypokalemia (2.1 mEq/L) and an inappropriately alkaline urine (pH 6.8).

In the setting of acidemia, normal kidneys excrete acid, resulting in a urine pH < 5.5. An alkaline urine pH > 5.5 with severe hypokalemia strongly points to a distal renal acidification defect.

4

Correlate the laboratory findings with the clinical history to establish the final diagnosis and mechanism.

The combination of hypokalemic NAGMA, alkaline urine, and a clinical history suggestive of Sjögren syndrome (dry eyes, dry mouth, rheumatoid arthritis) defines Type 1 (distal) Renal Tubular Acidosis, caused by an inability of alpha-intercalated cells to secrete hydrogen ions.

Sjögren syndrome is classically associated with autoantibody-mediated damage to the distal tubule, leading to impaired hydrogen ion secretion.

Key Concept

Type 1 (distal) renal tubular acidosis presents with a normal anion gap metabolic acidosis, severe hypokalemia, and an inappropriately alkaline urine (pH > 5.5). It is caused by impaired distal hydrogen ion secretion and is frequently associated with autoimmune disorders like Sjögren syndrome.

Question 168Question →

A 45-year-old man comes to the emergency department reporting four days of frequent, large-volume watery stools. He feels weak and lightheaded when standing. Physical examination reveals dry mucous membranes, flat neck veins, and delayed capillary refill. Laboratory results are as follows:

Test	Result
Sodium	$138$ mEq/L
Potassium	$3.2$ mEq/L
Chloride	$114$ mEq/L
Bicarbonate	$14$ mEq/L
Arterial pH	$7.28$
PaCO2	$30$ mm Hg

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

Normal anion gap metabolic acidosis

High anion gap metabolic acidosis

Metabolic alkalosis

Primary respiratory acidosis

Primary respiratory alkalosis

Show answer & explanation

Answer: Normal anion gap metabolic acidosis

Answer

Normal anion gap metabolic acidosis

The patient has a low pH (

7.28

) driven by a significantly decreased serum bicarbonate (

14

mEq/L), confirming a primary metabolic acidosis. The anion gap is calculated as

Na - (Cl + HCO_3)

, which equates to

138 - (114 + 14) = 10

mEq/L. Since this value is within the standard normal reference range (

8-12

mEq/L), the diagnosis is a normal anion gap metabolic acidosis. This is a classic presentation for a patient suffering from severe diarrhea, which causes profound gastrointestinal depletion of bicarbonate, ultimately leading to a compensatory retention of serum chloride (hyperchloremic metabolic acidosis).

Step-by-Step Solution

1

Analyze the arterial pH to determine the primary state.

The pH is

7.28

(less than

7.35

), indicating acidemia.

Identifying acidemia or alkalemia is the necessary first step in diagnosing any acid-base disorder.

2

Assess the serum bicarbonate and PaCO2 to identify the primary driver of the disorder.

The bicarbonate is low (

14

mEq/L), which directly aligns with the acidemic pH, indicating a primary metabolic acidosis. The low PaCO2 (

30

mm Hg) represents respiratory compensation.

Matching the direction of the abnormal pH to the corresponding metabolic (bicarbonate) or respiratory (PaCO2) parameter accurately identifies the primary disturbance.

3

Calculate the serum anion gap using the formula:

Na - (Cl + HCO_3)

.

138 - (114 + 14) = 138 - 128 = 10

mEq/L. This is within the normal range of

8-12

mEq/L.

Categorizing a confirmed metabolic acidosis as either normal or high anion gap narrows down the underlying etiology.

Key Concept

Severe diarrhea causes a normal anion gap (hyperchloremic) metabolic acidosis due to direct gastrointestinal loss of bicarbonate.

Question 169Question →

Match each set of laboratory findings with the most likely underlying etiology. Assume all patients are normotensive adults who are not currently taking any prescription medications.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Profile 1 matches with Distal (Type 1) RTA, Profile 2 matches with Hyporeninemic hypoaldosteronism (Type 4 RTA), Profile 3 matches with Surreptitious vomiting, and Profile 4 matches with Bartter syndrome.

Correct matching is achieved by systematically applying the diagnostic algorithms for acid-base disturbances. For normal anion gap metabolic acidosis (NAGMA), a positive urine anion gap indicates a renal acidification defect (RTA). A urine pH > 5.5 in the presence of acidemia points to a distal

H^+

secretion failure (Type 1 RTA), whereas a pH < 5.5 with hyperkalemia indicates defective ammoniagenesis due to aldosterone deficiency or resistance (Type 4 RTA). For metabolic alkalosis, urine chloride < 20 mEq/L defines volume-contracted (saline-responsive) states like vomiting, while urine chloride > 20 mEq/L in a normotensive patient with hypercalciuria points to a thick ascending limb defect, such as Bartter syndrome.

Step-by-Step Solution

1

Calculate the serum anion gap (

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

) for each profile.

Profile 1 (AG=10) and Profile 2 (AG=8) are normal anion gap metabolic acidoses (NAGMA). Profile 3 and Profile 4 are metabolic alkaloses.

This establishes the primary acid-base disturbance and narrows the differential diagnosis.

2

Evaluate the NAGMA profiles (1 and 2) using serum

K^+

, urine anion gap (

Na^+ + K^+ - Cl^-

), and urine pH.

Profile 1 has low

K^+

, a positive UAG (+45), and high urine pH (6.5), diagnostic of Type 1 RTA. Profile 2 has high

K^+

, a positive UAG (+20), and low urine pH (5.1), diagnostic of Type 4 RTA.

UAG distinguishes GI vs renal bicarbonate loss, and urine pH combined with

K^+

differentiates the RTA subtypes.

3

Evaluate the metabolic alkalosis profiles (3 and 4) using spot urine chloride.

Profile 3 has low urine

Cl^-

(8 mEq/L), indicating a saline-responsive state. Profile 4 has high urine

Cl^-

(110 mEq/L), indicating a saline-resistant state.

Urine chloride is the most accurate marker of volume status and pathophysiology in metabolic alkalosis.

4

Distinguish the specific etiologies for the alkalotic profiles based on associated findings.

Profile 3's high urine

Na^+

despite low

Cl^-

suggests active vomiting. Profile 4's high

Cl^-

and elevated urine calcium in a normotensive patient points to Bartter syndrome.

Active vomiting obligates

Na^+

excretion with bicarbonate, while Bartter syndrome mimics loop diuretic effects, leading to hypercalciuria.

Key Concept

Diagnosis of metabolic acid-base disorders requires systematic integration of serum anion gap, serum potassium, urine anion gap, and urine electrolytes.

Question 170Question →

A 62-year-old man is evaluated in the hospital for decreased urine output over the past 24 hours. He was admitted three days ago for the treatment of community-acquired pneumonia with intravenous antibiotics. His blood pressure is 110/70 mm Hg and his pulse is 88/min. Laboratory studies show:

Serum sodium: $140 \text{ mEq/L}$
Serum creatinine: $4.0 \text{ mg/dL}$
Urine sodium: $70 \text{ mEq/L}$
Urine creatinine: $80 \text{ mg/dL}$

Based on these laboratory values, what is this patient's fractional excretion of sodium (in percent)?

Show answer & explanation

Answer: 2.5

Answer

2.5

The correct calculation for the Fractional Excretion of Sodium (FENa) uses the formula

(U_{Na} \times P_{Cr}) / (P_{Na} \times U_{Cr}) \times 100

. Plugging in the given values gives

(70 \times 4.0) / (140 \times 80) \times 100 = 280 / 11200 \times 100 = 2.5\%

. A FENa greater than 2% in the setting of acute kidney injury is typically suggestive of acute tubular necrosis (ATN), which aligns with this patient's recent exposure to nephrotoxic intravenous antibiotics.

Step-by-Step Solution

1

Identify the mathematical formula for Fractional Excretion of Sodium (FENa).

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

This formula relates the clearance of sodium to the clearance of creatinine to determine the percentage of filtered sodium that is excreted.

2

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

FENa = \frac{70 \times 4.0}{140 \times 80} \times 100

Applying the specific serum and urine values provided in the clinical vignette.

3

Perform the calculation to find the final percentage.

FENa = \frac{280}{11200} \times 100 = 0.025 \times 100 = 2.5

Calculating the expression yields the final FENa percentage.

Key Concept

The Fractional Excretion of Sodium (FENa) is a calculated metric used to evaluate acute kidney injury, differentiating between prerenal (FENa < 1%) and intrinsic renal (FENa > 2%) etiologies.

Question 171Question →

A 63-year-old man is brought to the emergency department by ambulance after vomiting a large amount of bright red blood. He has a history of alcohol use disorder and cirrhosis. On arrival, he is confused and lethargic. Temperature is 36.5°C (97.7°F), blood pressure is 85/50 mm Hg, and pulse is 124/min. Physical examination shows scleral icterus, spider angiomas on the chest, and a distended abdomen with a positive fluid wave. Two large-bore intravenous lines are placed, and fluid resuscitation is initiated.

Laboratory studies obtained prior to fluid administration show:

Serum:
Sodium: 140 mEq/L
Blood urea nitrogen: 68 mg/dL
Creatinine: 2.1 mg/dL

Urine:
Sodium: 12 mEq/L
Creatinine: 60 mg/dL

Calculate the patient's fractional excretion of sodium (in percent).

Show answer & explanation

Answer: 0.3

Answer

0.3

The fractional excretion of sodium (FENa) is a useful calculation to distinguish prerenal acute kidney injury from intrinsic renal causes, such as acute tubular necrosis. A FENa of less than 1% typically indicates a prerenal state (e.g., hypovolemia or decreased effective circulating volume), where the kidneys appropriately and avidly reabsorb sodium to restore volume. In this patient with upper gastrointestinal bleeding and severe hypovolemia, the calculated FENa is

0.3\%

, which strongly supports a prerenal etiology for his acute renal impairment.

Step-by-Step Solution

1

Identify the required variables for the FENa formula from the laboratory results.

U_{Na}

= 12 mEq/L,

P_{Cr}

= 2.1 mg/dL,

P_{Na}

= 140 mEq/L,

U_{Cr}

= 60 mg/dL.

These specific serum and urine values are necessary to calculate the fractional excretion of sodium.

2

Set up the FENa formula.

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

This formula evaluates sodium clearance relative to creatinine clearance to estimate the percentage of filtered sodium that is excreted.

3

Perform the calculation.

FENa = \frac{12 \times 2.1}{140 \times 60} \times 100 = \frac{25.2}{8400} \times 100 = 0.003 \times 100 = 0.3

Mathematically solving the equation yields the final percentage.

Key Concept

Fractional Excretion of Sodium (FENa) Calculation

Question 172Question →

Match each clinical presentation and associated laboratory profile to the most likely underlying pathophysiologic mechanism.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Patient 1 matches impaired distal tubular secretion of hydrogen ions (Type 1 RTA). Patient 2 matches decreased production or effect of aldosterone (Type 4 RTA). Patient 3 matches profound gastrointestinal bicarbonate loss. Patient 4 matches gastric volume and hydrochloric acid depletion.

Each laboratory profile requires multi-step analysis: calculating the serum anion gap and, if normal, calculating the urine anion gap (

\text{UAG} = Na^+ + K^+ - Cl^-

). A negative UAG indicates intact renal ammonium excretion (e.g., gastrointestinal bicarbonate loss), whereas a positive UAG indicates impaired renal acidification (e.g., renal tubular acidosis). The profiles are further differentiated by serum potassium levels and urine pH. For metabolic alkalosis, the urine chloride level successfully differentiates saline-responsive from saline-resistant forms.

Step-by-Step Solution

1

Calculate the serum anion gap for each acid-base disturbance using the formula

\text{AG} = Na^+ - (Cl^- + HCO_3^-)

.

Patients 1, 2, and 3 have a normal serum anion gap (between

10-12

mEq/L), identifying normal anion gap metabolic acidosis (NAGMA). Patient 4 has metabolic alkalosis (

HCO_3^-

of

35

mEq/L).

Differentiates between high anion gap metabolic acidosis (HAGMA), NAGMA, and metabolic alkalosis to establish the primary disorder.

2

For patients with NAGMA (1, 2, and 3), calculate the urine anion gap using the formula

\text{UAG} = Na^+ + K^+ - Cl^-

.

Patient 1:

\text{UAG} = 45 + 20 - 35 = +30

. Patient 2:

\text{UAG} = 50 + 25 - 65 = +10

. Patient 3:

\text{UAG} = 30 + 15 - 75 = -30

.

A negative UAG indicates appropriate renal ammonium excretion (e.g., GI loss of

HCO_3^-

), while a positive UAG indicates impaired renal acid excretion (e.g., Renal Tubular Acidosis).

3

Evaluate serum potassium and urine pH for patients with positive UAG to differentiate RTA types.

Patient 1 has hypokalemia and a urine pH >

5.5

, indicating Type 1 RTA. Patient 2 has hyperkalemia and a urine pH <

5.5

, indicating Type 4 RTA.

Type 1 RTA involves a distal

H^+

secretion defect (high urine pH, hypokalemia). Type 4 involves aldosterone deficiency or resistance (low urine pH, hyperkalemia).

4

Evaluate the metabolic alkalosis in Patient 4 using the urine chloride level.

Patient 4 has a urine chloride of

8

mEq/L (which is <

20

mEq/L).

Low urine chloride signifies a chloride-responsive metabolic alkalosis, commonly due to gastric fluid loss (vomiting), supporting the clinical clues of lethargy and poor dentition.

Key Concept

Differentiation of normal anion gap metabolic acidosis (NAGMA) using the urine anion gap, serum potassium, and urine pH, alongside identifying chloride-responsive metabolic alkalosis via urine chloride levels.

Question 173Question →

A 19-year-old male is brought to the urgent care clinic by his parents due to extreme fatigue, increased thirst, and frequent urination over the past week. Today, he developed nausea and deep, rapid breathing. Laboratory evaluation shows:

Serum sodium: $138 \text{ mEq/L}$
Serum chloride: $104 \text{ mEq/L}$
Serum bicarbonate: $12 \text{ mEq/L}$

Arterial blood gas analysis on room air reveals:
pH: $7.24$
$PaCO_2$ : $26 \text{ mm Hg}$
$PaO_2$ : $95 \text{ mm Hg}$

Based on these findings, which of the following is the most accurate interpretation of this patient's acid-base status?

Primary metabolic acidosis with adequate respiratory compensation

Primary metabolic acidosis with concurrent respiratory acidosis

Primary metabolic acidosis with concurrent respiratory alkalosis

Primary respiratory acidosis with adequate metabolic compensation

Primary respiratory alkalosis with adequate metabolic compensation

Show answer & explanation

Answer: Primary metabolic acidosis with adequate respiratory compensation

Answer

Primary metabolic acidosis with adequate respiratory compensation

The correct answer accurately identifies the patient's condition. The pH of 7.24 indicates acidemia, and the low bicarbonate level of 12 mEq/L establishes metabolic acidosis as the primary disorder. To determine if the respiratory compensation is appropriate, Winter's formula is used: Expected

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

. For this patient, the calculation yields

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

. Since the patient's actual

PaCO_2

is exactly 26 mm Hg, the respiratory compensation is adequate, and there is no secondary acid-base disturbance.

Step-by-Step Solution

1

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

The pH is 7.24 (acidemia) and bicarbonate is 12 mEq/L (low).

A low pH combined with a proportionally low bicarbonate level confirms that the primary process is a metabolic acidosis.

2

Calculate the expected respiratory compensation using Winter's formula.

Expected

PaCO_2 = (1.5 \times 12) + 8 \pm 2 = 26 \pm 2 \text{ mm Hg}

.

In primary metabolic acidosis, the lungs rapidly compensate by blowing off

CO_2

(hyperventilation) to raise the pH. Winter's formula predicts the expected

PaCO_2

range for a given bicarbonate level.

3

Compare the actual

PaCO_2

to the expected

PaCO_2

.

The patient's actual

PaCO_2

is 26 mm Hg, which falls exactly within the expected range.

Because the actual

PaCO_2

matches the calculated value, the respiratory compensation is fully adequate, meaning there is no superimposed secondary respiratory disorder.

Key Concept

Applying Winter's formula to evaluate respiratory compensation in primary metabolic acidosis

Question 174Question →

A 42-year-old male patient is admitted to the intensive care unit for the management of severe acute pancreatitis. On presentation, he was tachycardic and hypotensive. Over the initial 24 hours of admission, he received 7 liters of 0.9% sodium chloride intravenously for aggressive fluid resuscitation. His vital signs are now stable. Laboratory studies obtained on hospital day 2 reveal the following:

Serum sodium: $142 \text{ mEq/L}$
Serum potassium: $4.2 \text{ mEq/L}$
Serum chloride: $115 \text{ mEq/L}$
Serum bicarbonate: $15 \text{ mEq/L}$
Blood urea nitrogen: $18 \text{ mg/dL}$
Serum creatinine: $0.9 \text{ mg/dL}$

Arterial blood gas on room air shows:
pH: $7.28$
$PaCO_2$ : $30 \text{ mmHg}$
$PaO_2$ : $92 \text{ mmHg}$

Which of the following is the most likely mechanism underlying this patient's current acid-base abnormality?

Intravenous administration of a fluid with a supraphysiologic chloride concentration

Hypoperfusion-induced shift to anaerobic metabolism and lactate production

Inadequate ventilatory response to a primary metabolic acid-base derangement

Failure of the renal tubules to reabsorb filtered bicarbonate

Retention of unmeasured uremic anions due to acute kidney injury

Show answer & explanation

Answer: Intravenous administration of a fluid with a supraphysiologic chloride concentration

Answer

Intravenous administration of a fluid with a supraphysiologic chloride concentration

This patient has a normal anion gap metabolic acidosis. The anion gap is calculated as

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

, which in this case is

142 - (115 + 15) = 12 \text{ mEq/L}

(normal is typically 8-12 mEq/L). The compensatory respiratory response is appropriate, as confirmed by Winter's formula: expected

PaCO_2 = (1.5 \times 15) + 8 = 30.5 \pm 2 \text{ mmHg}

, matching the measured value of 30 mmHg. The cause of this isolated, normal anion gap metabolic acidosis is the aggressive fluid resuscitation with 0.9% normal saline. Normal saline is actually supraphysiologic in its chloride content (154 mEq/L compared to a normal serum chloride of ~100 mEq/L). Massive infusion expands the extracellular volume and drives up serum chloride, which causes a relative decrease in serum bicarbonate to maintain electrical neutrality, resulting in hyperchloremic metabolic acidosis.

Step-by-Step Solution

1

Identify the primary acid-base disorder.

The pH is acidemic (7.28) and the serum bicarbonate is low (15 mEq/L), indicating a primary metabolic acidosis.

Establishing the primary disorder dictates which compensatory formulas and diagnostic algorithms to use.

2

Calculate the serum anion gap.

Anion Gap =

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

=

142 - (115 + 15)

=

12 \text{ mEq/L}

.

Metabolic acidosis must be classified into high anion gap or normal anion gap (hyperchloremic) to narrow the differential diagnosis.

3

Calculate the expected respiratory compensation.

Using Winter's formula: Expected

PaCO_2 = (1.5 \times 15) + 8 = 30.5 \pm 2

. The actual

PaCO_2

is 30 mmHg, which is perfectly compensated.

Checking compensation ensures there is no superimposed primary respiratory acid-base disorder.

4

Determine the cause of the normal anion gap metabolic acidosis.

The patient received 7 liters of 0.9% sodium chloride, which contains 154 mEq/L of chloride (supraphysiologic compared to the normal serum level of ~100 mEq/L), leading to hyperchloremic acidosis.

Correlating the calculated laboratory derangement with the clinical history provides the final diagnosis.

Key Concept

Normal saline (0.9% NaCl) contains 154 mEq/L of chloride. Large volume resuscitation with this fluid causes a hyperchloremic, normal anion gap metabolic acidosis.

Estimated Time:1m 0s

Question 175Question →

A 22-year-old woman is brought to the emergency department 4 hours after intentionally ingesting an unknown quantity of pills. She complains of ringing in her ears, nausea, and diffuse abdominal pain. Her temperature is 37.8°C (100.0°F), blood pressure is 110/70 mm Hg, pulse is 115/min, and respirations are 26/min.

Laboratory studies show:

Test	Value
Serum sodium	140 mEq/L
Serum potassium	3.5 mEq/L
Serum chloride	104 mEq/L
Serum bicarbonate	14 mEq/L
Blood urea nitrogen	18 mg/dL
Serum creatinine	0.9 mg/dL

Arterial blood gas analysis on room air:

Test	Value
pH	7.42
PaCO2	22 mm Hg
PaO2	98 mm Hg

Which of the following best characterizes this patient's primary acid-base disturbance(s)?

High anion gap metabolic acidosis with appropriate respiratory compensation

Mixed high anion gap metabolic acidosis and primary respiratory alkalosis

Mixed normal anion gap metabolic acidosis and primary respiratory alkalosis

Primary respiratory alkalosis with appropriate metabolic compensation

Mixed high anion gap metabolic acidosis and primary 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

This patient has a mixed acid-base disorder characteristic of acute salicylate toxicity (indicated by tinnitus, nausea, and hyperventilation). Step 1: The anion gap is

140 - (104 + 14) = 22

, confirming a high anion gap metabolic acidosis. Step 2: To check for appropriate respiratory compensation, Winter's formula is applied:

Expected PaCO_2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2

. Step 3: The patient's measured PaCO2 is 22 mm Hg, which is lower than the expected 27-31 mm Hg range. This indicates an excessive respiratory drive beyond what is needed for compensation, signifying an independent primary respiratory alkalosis. Step 4: The delta-delta ratio is

(22 - 12) / (24 - 14) = 1.0

, which rules out any additional primary metabolic alkalosis or non-anion gap metabolic acidosis.

Step-by-Step Solution

1

Calculate the anion gap to determine if a primary metabolic acidosis is present.

Anion Gap = Na^+ - (Cl^- + HCO_3^-) = 140 - (104 + 14) = 22 \text{ mEq/L}

.

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

2

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

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 expected range is 27 to 31 mm Hg.

Winter's formula determines if the patient's respiratory drive is appropriately compensating for the metabolic acidosis or if there is a secondary respiratory disorder.

3

Compare the measured PaCO2 to the expected PaCO2.

The measured

PaCO_2

is 22 mm Hg, which is lower than the expected range of 27-31 mm Hg.

Because the measured PaCO2 is lower than the expected appropriate compensation, an independent primary respiratory alkalosis is present.

4

Calculate the delta-delta ratio to evaluate for an additional covert metabolic disorder.

\Delta AG / \Delta HCO_3^- = (22 - 12) / (24 - 14) = 10 / 10 = 1.0

.

A ratio between 1 and 2 indicates a pure high anion gap metabolic acidosis without a concurrent primary metabolic alkalosis or non-anion gap metabolic acidosis.

Key Concept

Patients with salicylate toxicity typically present with a mixed high anion gap metabolic acidosis (due to uncoupling of oxidative phosphorylation) and a primary respiratory alkalosis (due to direct stimulation of the medullary respiratory center).

Question 176Question →

Match each clinical presentation with its most likely primary acid-base disorder.

Click a left item, then click its matching right item

Show answer & explanation

Answer

The presentations match as follows: missed insulin doses with High anion gap metabolic acidosis; COPD hypoventilation with Respiratory acidosis; panic attack hyperventilation with Respiratory alkalosis; intractable vomiting with Metabolic alkalosis.

Each clinical scenario describes a classic, straightforward presentation for one of the four primary acid-base disorders. Identifying the fundamental underlying mechanism—such as ketoacid overproduction, carbon dioxide retention, carbon dioxide washout, or direct acid loss—is essential for accurate diagnosis.

Step-by-Step Solution

1

Analyze the patient with type 1 diabetes and severe hyperglycemia.

The clinical picture indicates diabetic ketoacidosis.

This condition is characterized by the production of ketoacids, which are unmeasured anions that consume bicarbonate and widen the anion gap.

2

Analyze the patient with a severe COPD exacerbation and hypoventilation.

The decreased respiratory drive leads to carbon dioxide retention.

Retained carbon dioxide acts directly as an acid in the blood, leading to a primary respiratory acidosis.

3

Analyze the patient with acute hyperventilation from a panic attack.

The rapid breathing causes excessive elimination of carbon dioxide.

Loss of carbon dioxide raises the blood pH, resulting in a primary respiratory alkalosis.

4

Analyze the patient with intractable vomiting.

Vomiting leads to the continuous loss of gastric contents.

Gastric fluid is rich in hydrochloric acid. Losing this acid directly increases the serum pH, causing a primary metabolic alkalosis.

Key Concept

Primary Acid-Base Disorders

Question 177Question →

Match each clinical presentation and disease association to the most likely corresponding nephrotic syndrome etiology.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Solid tumors correlate with Membranous nephropathy, HIV infection with FSGS, Hepatitis C with MPGN, and childhood post-viral onset with Minimal change disease.

Each type of nephrotic syndrome has classic epidemiological and systemic associations. Identifying these triggers (e.g., solid tumors, specific viral infections, or age demographics) is critical for determining the specific glomerular pathology and guiding subsequent management.

Step-by-Step Solution

1

Analyze the adult presentation with a solid tumor and thickened capillary loops.

This profile maps to Membranous nephropathy.

Membranous nephropathy is the classic nephrotic syndrome paraneoplastic manifestation of solid organ malignancies.

2

Analyze the patient with HIV and focal sclerotic lesions.

This profile maps to Focal segmental glomerulosclerosis (FSGS).

HIV-associated nephropathy (HIVAN) is a highly specific, rapidly progressive form of collapsing FSGS.

3

Analyze the Hepatitis C patient with tram-track glomerular splitting.

This profile maps to Membranoproliferative glomerulonephritis (MPGN).

Hepatitis C drives immune-complex deposition in the glomeruli, characteristic of type I MPGN.

4

Analyze the pediatric patient with post-viral edema.

This profile maps to Minimal change disease.

The sudden onset of pure nephrotic syndrome in a child after a viral illness is the hallmark of Minimal change disease.

Key Concept

Secondary causes and systemic associations of nephrotic syndromes

Question 178Question →

A 54-year-old man with a history of severe chronic obstructive pulmonary disease and major depressive disorder is brought to the emergency department by paramedics after being found unarousable in his garage. An open, empty container of antifreeze was found nearby. Vital signs are temperature 36.7°C (98.1°F), blood pressure 112/68 mmHg, pulse 110/min, and respirations 16/min. Pulse oximetry is 89% on room air. Physical examination reveals an obtunded patient with a barrel-shaped chest, diminished breath sounds, and diffuse bilateral wheezing.

Laboratory testing yields the following results:

Laboratory Test	Value
Sodium	142 mEq/L
Potassium	4.8 mEq/L
Chloride	106 mEq/L
Bicarbonate	12 mEq/L
Arterial pH	7.21
Arterial $pCO_2$	36 mmHg
Arterial $pO_2$	62 mmHg

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

Primary metabolic acidosis with appropriate respiratory compensation

Primary respiratory acidosis with appropriate metabolic compensation

Mixed metabolic acidosis and respiratory acidosis

Mixed metabolic acidosis and respiratory alkalosis

Mixed metabolic acidosis and metabolic alkalosis

Show answer & explanation

Answer: Mixed metabolic acidosis and respiratory acidosis

Answer

Mixed metabolic acidosis and respiratory acidosis

The correct answer accurately identifies two distinct acid-base disorders in this patient. First, the low pH (7.21) and low bicarbonate (12 mEq/L) with an elevated anion gap of 24 mEq/L confirm a high anion gap metabolic acidosis, consistent with toxic alcohol ingestion. Second, applying Winter's formula dictates that his expected

pCO_2

should be between 24 and 28 mmHg. Because his actual

pCO_2

is 36 mmHg, he is retaining more

CO_2

than he should, indicating an independent, concurrent respiratory acidosis caused by his baseline severe COPD.

Step-by-Step Solution

1

Determine the primary acid-base disorder by looking at the pH,

pCO_2

, and

HCO_3^-

.

The pH is 7.21 (acidemia). The

HCO_3^-

is 12 mEq/L (low). This establishes a primary metabolic acidosis.

Acidemia driven by a low bicarbonate indicates a primary metabolic acidosis, which in this clinical context is likely due to ethylene glycol toxicity.

2

Calculate the anion gap to further characterize the metabolic acidosis.

AG = Na^+ - (Cl^- + HCO_3^-) = 142 - (106 + 12) = 142 - 118 = 24

mEq/L.

An elevated anion gap (normal 8-12 mEq/L) confirms a high anion gap metabolic acidosis (HAGMA).

3

Calculate the expected respiratory compensation using Winter's formula.

Expected

pCO_2 = (1.5 \times 12) + 8 \pm 2 = 18 + 8 \pm 2 = 26 \pm 2

mmHg (Range: 24 to 28 mmHg).

Winter's formula determines the appropriate ventilatory response to a metabolic acidosis.

4

Compare the actual

pCO_2

to the expected

pCO_2

to identify any secondary respiratory disorders.

The actual measured

pCO_2

is 36 mmHg. Since 36 mmHg is greater than the upper limit of the expected range (28 mmHg), the patient is hypoventilating relative to their acidemic state.

A

pCO_2

higher than the expected compensatory range indicates a concurrent primary respiratory acidosis, likely due to this patient's severe underlying COPD limiting his ability to appropriately blow off

CO_2

.

Key Concept

In patients with metabolic acidosis, Winter's formula (

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

) must be used to calculate expected

pCO_2

. If the measured

pCO_2

is significantly higher than expected, a concurrent respiratory acidosis is present.

Question 179Question →

A 65-year-old male with poorly controlled type 2 diabetes mellitus presents to the emergency department following 3 days of intractable vomiting, diffuse abdominal pain, and an inability to tolerate oral intake. He reports that he stopped taking his prescribed insulin regimen 48 hours ago due to his lack of appetite. On physical examination, he appears lethargic and mildly confused. His mucous membranes are completely dry, and he has markedly decreased skin turgor. His temperature is 37.1°C (98.8°F), blood pressure is 90/60 mm Hg, pulse is 112/min, and respiratory rate is 24/min.

Laboratory studies show the following:
Sodium: $142 \text{ mEq/L}$
Potassium: $3.8 \text{ mEq/L}$
Chloride: $94 \text{ mEq/L}$
Bicarbonate: $16 \text{ mEq/L}$
Blood urea nitrogen: $45 \text{ mg/dL}$
Serum creatinine: $2.2 \text{ mg/dL}$
Glucose: $550 \text{ mg/dL}$
Serum ketones: large

Assuming a normal serum anion gap of $12 \text{ mEq/L}$ and a normal serum bicarbonate of $24 \text{ mEq/L}$ , what is the delta ratio ( $\Delta\text{AG} / \Delta\text{HCO}_3^-$ ) for this patient?

Show answer & explanation

Answer: 2.5

Answer

2.5

The delta ratio (

\Delta\text{AG} / \Delta\text{HCO}_3^-

) is calculated by first determining the anion gap, which is

142 - (94 + 16) = 32 \text{ mEq/L}

. The change in the anion gap (

\Delta\text{AG}

) is

32 - 12 = 20

. The change in bicarbonate (

\Delta\text{HCO}_3^-

) is

24 - 16 = 8

. Dividing the

\Delta\text{AG}

by the

\Delta\text{HCO}_3^-

gives

20 / 8 = 2.5

. In the clinical context of diabetic ketoacidosis (which produces a high anion gap metabolic acidosis), a delta ratio greater than

2.0

confirms the presence of an underlying concurrent metabolic alkalosis. This physiological state is perfectly consistent with this patient's history of severe, intractable vomiting, which causes a profound loss of gastric hydrochloric acid and volume contraction.

Step-by-Step Solution

1

Calculate the patient's anion gap (AG).

\text{AG} = 142 - (94 + 16) = 32 \text{ mEq/L}

The anion gap is calculated using the formula:

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

.

2

Calculate the delta anion gap (

\Delta\text{AG}

).

\Delta\text{AG} = 32 - 12 = 20

\Delta\text{AG}

is the calculated anion gap minus the normal expected anion gap (

12 \text{ mEq/L}

).

3

Calculate the delta bicarbonate (

\Delta\text{HCO}_3^-

).

\Delta\text{HCO}_3^- = 24 - 16 = 8

\Delta\text{HCO}_3^-

is the normal expected bicarbonate (

24 \text{ mEq/L}

) minus the patient's measured bicarbonate.

4

Calculate the delta ratio.

\Delta\text{Ratio} = \frac{20}{8} = 2.5

The delta ratio evaluates the relationship between the rise in unmeasured anions and the fall in bicarbonate. Dividing

\Delta\text{AG}

by

\Delta\text{HCO}_3^-

yields

2.5

.

Key Concept

In complex mixed acid-base disorders, calculating the delta ratio (

\Delta\text{AG} / \Delta\text{HCO}_3^-

) helps uncover hidden metabolic derangements. A delta ratio

> 2.0

suggests a concurrent metabolic alkalosis superimposed on a high anion gap metabolic acidosis.

Estimated Time:1m 30s

Question 180Question →

A 24-year-old woman with a history of type 1 diabetes mellitus presents to the emergency department with nausea, vomiting, and diffuse abdominal pain. She reports missing her insulin doses for the past two days due to illness. On physical examination, she appears lethargic. Her blood pressure is 105/65 mm Hg, heart rate is 118/min, and respiratory rate is 26/min with deep, rapid breaths. Laboratory studies show the following:

Laboratory test	Result
Serum sodium	140 mEq/L
Serum potassium	5.2 mEq/L
Serum chloride	98 mEq/L
Serum bicarbonate	12 mEq/L
Blood glucose	485 mg/dL
Serum creatinine	1.1 mg/dL

What is the patient's calculated serum anion gap? (Provide the numerical value in mEq/L).

Show answer & explanation

Answer: 30

Answer

30

The serum anion gap is calculated using the standard formula:

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

. In this patient, substituting the given laboratory values yields:

140 - (98 + 12) = 140 - 110 = 30

mEq/L. A normal anion gap is typically between 8 and 12 mEq/L. This patient has a significantly elevated anion gap of 30 mEq/L, which, in the context of her hyperglycemia and missed insulin doses, is highly consistent with diabetic ketoacidosis, a classic cause of high anion gap metabolic acidosis.

Step-by-Step Solution

1

Identify the standard formula for calculating the serum anion gap.

Anion Gap =

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

The anion gap is a calculated value used to differentiate the causes of metabolic acidosis by estimating the unmeasured anions in the serum.

2

Extract the relevant serum electrolyte values from the provided laboratory results.

Na^+ = 140

mEq/L,

Cl^- = 98

mEq/L, and

HCO_3^- = 12

mEq/L

These three specific electrolyte values are the components required for the standard anion gap calculation. Serum potassium is generally omitted from this formula on the USMLE.

3

Substitute the values into the formula and perform the calculation.

Anion Gap =

140 - (98 + 12) = 140 - 110 = 30

mEq/L

This mathematical operation yields the final numerical value of the patient's anion gap.

Key Concept

Calculation of the serum anion gap is a fundamental step in evaluating a patient with metabolic acidosis, utilizing the formula: Anion Gap =

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

.

Nephrology

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept