This is the answer and explanation for a USMLE practice question from the Dr. Ray Makar Facebook page. Try it first, then check your answer below.
The Question
A 34-year-old woman has vomited repeatedly for 4 days. ABG: pH 7.52, PaCO2 48 mm Hg, HCO3– 38 mEq/L. Serum K+ is 2.9 mEq/L and urine chloride is low. Which best describes her acid-base status?
A. Respiratory acidosis with metabolic compensation
B. Metabolic alkalosis with appropriate respiratory compensation
C. Mixed metabolic and respiratory alkalosis
D. Primary respiratory alkalosis
E. Metabolic acidosis with respiratory compensation
Correct Answer: B. Metabolic alkalosis with appropriate respiratory compensation
pH 7.52 is alkalemia, and the high HCO3– means the primary problem is metabolic alkalosis from loss of stomach acid. Expected PaCO2 = 0.7 × HCO3– + 21 = 0.7 × 38 + 21 ≈ 48 mm Hg, which matches, so the respiratory compensation is appropriate. Low urine chloride means it is saline-responsive: treat with normal saline and potassium.
Why the Other Answers Are Wrong
- Respiratory acidosis: The pH is high (alkalemia), not low.
- Mixed alkalosis: A mixed disorder would show a PaCO2 lower than expected; hers matches the expected value.
- Respiratory alkalosis: That would cause a low PaCO2, not a high one.
- Metabolic acidosis: HCO3– would be low.
High-Yield Pearl
Metabolic alkalosis compensation: expected PaCO2 = 0.7 × HCO3– + 21 (± 2). Low urine Cl– = vomiting (saline-responsive).
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