A 6-week-old male infant presents with non-bilious projectile vomiting after every feed. He is eager to feed again after vomiting. On examination there is a palpable olive-shaped mass in the right hypochondrium. The investigation of choice to confirm the diagnosis is:

• A Contrast upper GI series showing 'string sign' and 'shoulder sign'
• B Serum ammonia to exclude metabolic disorder
• C Abdominal ultrasound demonstrating pyloric muscle thickness ≥4 mm and pyloric channel length ≥16 mm ✓
• D CT scan of abdomen with contrast

Explanation

The clinical presentation describes hypertrophic pyloric stenosis (HPS) — the most common cause of non-bilious projectile vomiting in the first 2–8 weeks of life, predominantly in male infants. Ultrasound is the investigation of choice, demonstrating a pyloric muscle wall thickness ≥4 mm and pyloric channel length ≥16 mm. Contrast studies (string sign/shoulder sign) were the older diagnostic standard but are no longer first-line. The electrolyte disturbance is hypochloraemic hypokalaemic metabolic alkalosis (not ammonia). CT is inappropriate for this age group due to radiation.

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.