Paediatricsmiddle twist
A 6-day-old boy born at term is brought to the emergency department after vomiting three times over four hours; the most recent vomitus was dark green. He fed normally until this morning, passed meconium within 24 hours of birth, and has had normal wet and dirty nappies since. He is quiet with a heart rate of 172/min; his abdomen is soft and not distended, with no palpable masses, and the inguinal regions are normal. An abdominal X-ray shows a non-specific bowel gas pattern. Which one of the following is the most appropriate next investigation?