A 3-week-old male infant presents with progressively worsening non-bilious projectile vomiting after every feed for 10 days. Examination reveals visible gastric peristalsis and an olive-shaped mass in the right upper abdomen. Electrolytes show: Na 133, K 2.8, Cl 85, HCO3 32 mEq/L. What is the metabolic derangement and its cause?
- A Hypochloraemic hypokalaemic metabolic alkalosis due to loss of HCl in gastric vomitus ✓
- B Metabolic acidosis due to loss of alkaline intestinal secretions
- C Hyperchloraemic metabolic acidosis from renal tubular acidosis
- D Respiratory alkalosis from hyperventilation
Explanation
Hypertrophic pyloric stenosis classically presents in a 2–8-week-old first-born male with projectile non-bilious vomiting, olive-shaped epigastric mass, and hypochloraemic hypokalaemic metabolic alkalosis. The gastric vomitus contains HCl and KCl; loss of gastric acid (HCl) depletes chloride and hydrogen ions, causing metabolic alkalosis and hypochloraemia. Hypokalaemia occurs from secondary hyperaldosteronism (volume depletion triggers aldosterone) causing renal potassium wasting. The paradoxical aciduria despite alkalosis is due to the kidney preferentially excreting H⁺ and K⁺ in renal tubules to retain Na⁺. Pre-operative electrolyte and fluid correction is essential before Ramstedt pyloromyotomy.
Reference: Ghai Essential Pediatrics, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.