Q17
Single choice
A 4-year-old previously healthy girl presents to the emergency department with a 24-hour history of rectal bleeding and dizziness. She has no other gastrointestinal symptoms. On examination, she appears pale. Her heart rate is 140 beats/ min, and she has a 20 mmHg postural drop in systolic blood pressure. The child's abdomen is nondistended and nontender, and fresh blood and clots are in the rectal vault on rectal examination.
Which of the following is the most appropriate diagnostic study to order for this patient?