A 3-week-old male infant presents with progressive non-bilious projectile vomiting after feeding. Serum electrolytes: Na+ 136, K+ 2.8, Cl- 88, HCO3- 32 mEq/L. What is the correct pathophysiology of the electrolyte disturbance?

• A Loss of bile leads to metabolic acidosis with compensatory hyperventilation
• B Loss of gastric HCl leads to metabolic alkalosis; renal compensation via paradoxical aciduria ✓
• C Pyloric obstruction causes proximal small bowel fluid loss with metabolic acidosis
• D Hyponatremia drives aldosterone suppression causing secondary hypokalemia

Explanation

Hypertrophic pyloric stenosis causes loss of gastric secretions (HCl) leading to hypochloremic hypokalemic metabolic alkalosis. Initially, the kidney compensates by excreting bicarbonate with sodium; as dehydration and sodium depletion worsen, the kidney prioritizes sodium retention over acid-base correction, resulting in paradoxical aciduria (acidic urine despite systemic alkalosis), as Na+ is reabsorbed in exchange for H+ rather than HCO3-. Correction involves resuscitation with normal saline + potassium before surgical pyloromyotomy (Ramstedt procedure).

Reference: Bailey & Love's Short Practice of Surgery, 27th ed.

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