A 32-year-old woman is evaluated in the clinic for progressive generalized muscle weakness… — USMLE Step 2 CK

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

Laboratory results are as follows:

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

Arterial blood gas (room air):

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

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

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

Normal anion gap metabolic acidosis with appropriate respiratory compensation due to a distal tubular defectAnswer
B
High anion gap metabolic acidosis with appropriate respiratory compensation due to unmeasured endogenous acids
C
Mixed normal anion gap metabolic acidosis and primary respiratory alkalosis due to a proximal tubular defect
D
Mixed normal anion gap metabolic acidosis and primary respiratory acidosis due to gastrointestinal bicarbonate loss
E
Normal anion gap metabolic acidosis with inadequate respiratory compensation due to reduced adrenal aldosterone secretion

Answer

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

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

14 \text{ mEq/L}

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

10 \text{ mEq/L}

. Using Winter's formula, the expected

PaCO_2

is calculated as

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

. Because the actual

PaCO_2

is exactly

29 \text{ mm Hg}

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

Step-by-Step Solution

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

The low pH (

7.27

) and low serum bicarbonate (

14 \text{ mEq/L}

) confirm a primary metabolic acidosis.

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

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

The calculated AG is

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

, which falls within the normal range (

10-14 \text{ mEq/L}

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

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

Expected

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

. The patient's actual

PaCO_2

29 \text{ mm Hg}

, confirming perfect respiratory compensation with no secondary respiratory disorder.

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

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

The presence of severe hypokalemia (

2.7 \text{ mEq/L}

), an inappropriately alkaline urine (pH

6.5

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

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

Key Concept

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

Estimated Time:1m 30s

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