Pyloric stenosis — RACGP Fellowship MCQ

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HardPaediatrics and adolescentPyloric stenosisRACGP Fellowship

A 6-week-old infant presents with projectile non-bilious vomiting after feeds, weight loss, and persistent hunger despite vomiting. Examination reveals mild dehydration. Serum chloride is low and metabolic alkalosis is confirmed. What is the most likely diagnosis?

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Correct answer: A — Hypertrophic pyloric stenosis

The combination of age 6 weeks, projectile non-bilious vomiting after feeds with persistent hunger, weight loss, mild dehydration, and hypochloraemic metabolic alkalosis is diagnostic for hypertrophic pyloric stenosis (HPS). The hypochloraemia and metabolic alkalosis result from loss of gastric HCl through sustained vomiting, a hallmark biochemical finding. GORD does not cause projectile vomiting or characteristic metabolic derangement. Intussusception typically presents later (6–36 months) with colicky pain and bilious vomiting. Malrotation with volvulus presents acutely and catastrophically with bilious vomiting and shock. Cow's milk protein allergy causes feeding intolerance and diarrhoea, not projectile vomiting or hypochloraemic alkalosis. Ultrasound would confirm pyloric muscle hypertrophy and guide surgical management.

Reference: Pyloric stenosis: a retrospective study of an Australian population (2009, Royal Children's Hospital Brisbane); Presentation and outcomes in hypertrophic pyloric stenosis: An 11-year review (2019, Monash University, Melbourne). https://pubmed.ncbi.nlm.nih.gov/19840091/