Paediatricsmiddle twist
A 4-day-old term boy, discharged home yesterday and feeding well until this morning, has vomited four times in six hours. The vomit is dark green. He passed meconium at eight hours of age and had a normal soft stool an hour ago. He is alert, afebrile and well perfused, and the abdomen is soft and not distended. Blood glucose and venous gas are normal. Which one of the following investigations should be arranged urgently?