A 3-week-old male infant presents with non-bilious projectile vomiting after every feed, visible left-to-right peristaltic wave, and a palpable 'olive-shaped' mass in the right upper quadrant. Ultrasound confirms pyloric channel length >16 mm and pyloric wall thickness >4 mm. Prior to surgical pyloromyotomy (Ramstedt procedure), which metabolic abnormality must be corrected?

• A Hyperchloraemic metabolic acidosis
• B Hypokalaemic hypochloraemic metabolic alkalosis ✓
• C Hyponatraemic metabolic alkalosis
• D Dilutional metabolic acidosis from vomiting

Explanation

Infantile hypertrophic pyloric stenosis causes repeated loss of gastric secretions (HCl and K+), resulting in hypochloraemic, hypokalaemic metabolic alkalosis with paradoxical aciduria (kidney excretes H+ to conserve Na+). Pyloromyotomy is not a surgical emergency; the infant must be resuscitated with intravenous 0.9% NaCl with KCl supplementation until the metabolic alkalosis is corrected (serum Cl- >100 mEq/L, serum HCO3- <30 mEq/L) before anaesthesia is safe. Proceeding to surgery with uncorrected alkalosis risks apnoea post-anaesthesia due to respiratory centre suppression. Metabolic acidosis is not the characteristic abnormality here.

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

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Written and medically reviewed by the StethoPrep medical team.