A 73-year-old man presented with acute lower gastrointestinal bleeding. He was hemodynamically stable, and nasogastric-tube lavage revealed bile with no blood, suggesting a lower gastrointestinal source for the bleeding. Urgent colonoscopy, performed after a rapid colonic purge, revealed a profusely bleeding diverticulum in the sigmoid colon; at the neck of the diverticulum, a bleeding vessel was identified (arrow in Panel A). Hemostasis was achieved by injecting epinephrine (dilution, 1:10,000) around the diverticulum (Panel B). The patient was discharged two days later, without any further bleeding or complications. Diverticula develop where vasa recta (intramural branches of the marginal artery supplying the . . .