Paediatricscommon
An 8-month-old previously well boy has had a 12-hour history of episodes in which he suddenly screams, draws his knees up to his chest and goes pale, lasting a few minutes and recurring every 15–20 minutes; between episodes he is unusually quiet and floppy. He has vomited three times, most recently green fluid. His last nappy contained mucus mixed with blood, "like red jam". On examination he is lethargic, with a soft abdomen and a sausage-shaped mass palpable in the right upper quadrant; the right iliac fossa feels empty.
Which one of the following is the most appropriate initial investigation?