A 60-year-old man complains of recurrent attacks of painless rectal bleeding. Colonoscopy reveals normal mucosa between the cecum and the anal verge. What is the most helpful test to determine the cause of bleeding?

A common cause of lower GI bleeding that is recurrent and painless is angiodysplasia of the colon. In the absence of diverticula or hemorrhoids, the suspicion is even higher for these lesions. Peptic ulcer and Meckel's diverticulum can cause predominantly lower GI bleeding. However, the bleeding is usually in the form of melena rather than bright red.