A 3-week-old male infant presents with progressive projectile non-bilious vomiting after every feed for 5 days. On examination, an olive-shaped mass is palpable in the right epigastrium. Serum electrolytes: Na 138, Cl 88, K 3.0, HCO3 32 mEq/L. The metabolic derangement present is:
- A Hyperchloremic metabolic acidosis
- B Hyponatremic metabolic acidosis
- C Respiratory alkalosis with mild hypokalemia
- D Hypochloremic, hypokalemic metabolic alkalosis with paradoxical aciduria ✓
Explanation
Hypertrophic pyloric stenosis (HPS) causes repeated vomiting of gastric contents (HCl), leading to loss of H⁺ and Cl⁻. This results in hypochloremic, hypokalemic metabolic alkalosis. The kidneys initially excrete NaHCO₃ to correct alkalosis, but as hypovolemia worsens, the kidney prioritizes Na⁺ conservation (aldosterone effect) over HCO₃ excretion — the distal tubule exchanges H⁺ and K⁺ for Na⁺, producing an acid urine despite systemic alkalosis ('paradoxical aciduria'). Hypokalemia worsens as K⁺ is exchanged for Na⁺ in the distal tubule. Correction of the metabolic derangement with IV normal saline and KCl must occur before surgery.
Reference: Ghai Essential Pediatrics, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.