A 3-week-old male infant presents with projectile non-bilious vomiting after feeds. Examination reveals a palpable 'olive' in the right upper quadrant. Ultrasound confirms pyloric muscle thickness of 4 mm and channel length of 18 mm. What is the electrolyte abnormality and the immediate preoperative priority before Ramstedt's pyloromyotomy?

• A Hyperkalaemic metabolic acidosis — correct with sodium bicarbonate
• B Hypernatraemic dehydration — correct with 0.45% saline slowly over 48 hours
• C Hypoglycaemia — correct with dextrose and proceed urgently to surgery
• D Hypochloraemic, hypokalaemic metabolic alkalosis — correct with IV 0.9% saline with potassium supplementation before surgery ✓

Explanation

Hypertrophic pyloric stenosis causes loss of hydrochloric acid (HCl) in the gastric juice, leading to hypochloraemic, hypokalaemic metabolic alkalosis — the classic electrolyte disturbance. Hydrogen and chloride ions are lost; compensatory bicarbonate retention and renal potassium wasting (paradoxical aciduria) occur. Ramstedt's pyloromyotomy is an elective (not emergency) operation that must be delayed until metabolic correction with IV 0.9% saline plus potassium supplementation achieves serum chloride >100 mEq/L, sodium >135 mEq/L, and urine output >1 mL/kg/h.

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

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Written and medically reviewed by the StethoPrep medical team.