A 35-year-old woman presents with hypokalaemia (K+ 2.9 mEq/L), metabolic alkalosis (HCO3− 32 mEq/L), normal blood pressure, and low urinary chloride (< 10 mEq/L) after prolonged vomiting. What is the underlying pathophysiology?
- A Contraction alkalosis with chloride-responsive metabolic alkalosis ✓
- B Primary hyperaldosteronism
- C Type 1 renal tubular acidosis
- D Bartter syndrome
Correct answer: A. Contraction alkalosis with chloride-responsive metabolic alkalosis
Explanation
Prolonged vomiting causes loss of hydrochloric acid, generating metabolic alkalosis. Subsequent volume contraction stimulates aldosterone, causing renal potassium and hydrogen ion wasting which perpetuates the alkalosis. Urinary chloride < 10 mEq/L indicates chloride-responsive metabolic alkalosis (HCl loss, volume depletion), correctable with saline and KCl replacement. Primary hyperaldosteronism causes hypertension and urinary Cl > 20 mEq/L. Bartter syndrome also has urinary Cl > 20 with normal BP. RTA causes metabolic acidosis, not alkalosis.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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