A 13 week old male infant presented to his general practitioner with a 12 hour history of non-bilious vomiting and lethargy. He was previously well, with an uneventful perinatal period, and he had received his three month routine immunisations the previous day (diphtheria, tetanus, pertussis, polio, Haemophilus influenzae type b, meningitis C, and rotavirus). He was bottle fed and growing well. The vomiting started suddenly, and as the day progressed he became more lethargic. He was not crying or irritable, had no fever, and had opened his bowels normally in the past 24 hours with no blood or mucous. A urine sample was extremely cloudy but dipstick testing was negative. He was referred to the paediatric emergency department. In the emergency department his observations were within the normal range for his age, but he appeared pale, with a prolonged capillary refill time of 3 seconds. Abdominal examination showed some possible upper abdominal tenderness but no obvious masses or distension, and normal bowel sounds were heard. Clinically he was well hydrated but was very quiet, and while in the department he had a single episode of bright green (bilious) vomiting. A plain abdominal radiograph showed dilated loops of small bowel but nothing specific (fig 1⇓). Fig 1 Plain abdominal radiograph showing dilated loops of small bowel ### 1. What is the differential diagnosis? #### Short answer Although sepsis should always be considered in an unwell infant, bilious vomiting is a sign of intestinal obstruction until proved otherwise. The most common cause of obstruction in infants and children aged 3 months to 6 years is intussusception,1 but other causes include obstruction secondary to an inguinal hernia and volvulus due to malrotation.2 …