MSFinals-4098

A 3-year-old girl is brought to the paediatric emergency department by her father. She has been very fussy for the past 6 hours, crying out every 20 minutes, and her father suspects she is in pain. She has vomited twice during this time, and her father noticed a red jelly-like substance in her diaper 2 hours ago. The little girl is very uncooperative during examination and refuses to let anyone touch her abdomen. Her vital signs show a mild tachycardia, but no fever is present. What is the most appropriate diagnostic test to determine the cause of her symptoms?