A 42-year-old man is evaluated in the emergency department for a 2-day history of bright red blood per rectum. He reports feeling lightheaded, which started this morning and prompted him to come to the emergency department. His medical history is otherwise unremarkable.
On physical examination, temperature is 37.0 °C (98.6 °F), blood pressure is 95/60 mm Hg, pulse rate is 110/min, and respiration rate is 20/min. He is clammy. Abdominal examination findings are normal. Rectal examination shows fresh blood.
He is admitted to the hospital and is stabilized with fluid resuscitation and erythrocyte transfusions.
Laboratory studies show a hemoglobin level of 8.5 g/dL (85 g/L). Upper endoscopy results are normal. Colonoscopy shows fresh blood throughout the colon and terminal ileum.
He stabilizes overnight, but the next morning he has several large bowel movements of fresh blood. He is hemodynamically unstable, and fluid resuscitation is initiated.
Which of the following is the most appropriate test to perform next?