A 3-week-old male infant presents with progressive non-bilious projectile vomiting after every feed. Examination reveals visible peristalsis and a palpable 'olive' in the epigastrium. Ultrasound confirms pyloric muscle thickness of 4 mm and channel length of 18 mm. What metabolic abnormality is expected on blood gas?

• A Metabolic acidosis with hyperkalaemia
• B Metabolic alkalosis with hypochloraemia and hypokalaemia ✓
• C Respiratory alkalosis with hypernatraemia
• D Normal electrolytes with mild dehydration

Explanation

Hypertrophic pyloric stenosis causes repeated loss of gastric HCl, resulting in hypochloraemic, hypokalaemic metabolic alkalosis with paradoxical aciduria (the kidney preferentially excretes H+ to conserve K+). Resuscitation with normal saline and potassium correction must achieve normalisation of electrolytes BEFORE surgery (Ramstedt's pyloromyotomy), as the operation is semi-urgent, not emergent — the metabolic derangement is the immediate threat, not the obstruction.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.