A 3-week-old male infant presents with non-bilious projectile vomiting. Ultrasound of the pylorus shows a pyloric muscle thickness of 4.5 mm and channel length of 18 mm. The diagnosis is:

• A Duodenal atresia
• B Antral web
• C Gastroesophageal reflux
• D Pyloric stenosis ✓

Explanation

Hypertrophic pyloric stenosis is diagnosed on ultrasound when muscle thickness is ≥4 mm (positive predictive value ~100%) and pyloric channel length is ≥16 mm. Non-bilious vomiting in a 3–6 week male infant is the classic clinical setting. Duodenal atresia produces bilious vomiting and the double-bubble sign. An antral web would show a filling defect, not muscle thickening.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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