A 3-week-old male infant presents with non-bilious projectile vomiting occurring immediately after feeds, visible peristalsis, and a palpable olive-shaped mass in the right upper quadrant. The electrolyte finding expected on blood gas is:

• A Hyperchloraemic metabolic acidosis
• B Respiratory alkalosis with metabolic compensation
• C Hypochloraemic hypokalaemic metabolic alkalosis ✓
• D Normal acid-base with hyponatraemia

Explanation

Hypertrophic pyloric stenosis causes prolonged vomiting of gastric (HCl-rich) contents, leading to loss of H+ and Cl- ions. The kidneys compensate by retaining Na+ at the expense of K+ and H+ (paradoxical aciduria), producing hypochloraemic, hypokalaemic metabolic alkalosis. Pre-operative correction of this electrolyte imbalance is mandatory before pyloromyotomy (Ramstedt's operation) to avoid anaesthetic complications.

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

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Written and medically reviewed by the StethoPrep medical team.