PLE-8321

A 77-year-old woman is brought to the emergency department after collapsing at home. Six hours ago, she had the sudden onset of massive bright red rectal bleeding. On arrival, her blood pressure is 90/60 mm Hg, and pulse is 120/min. Abdominal examination shows no abnormalities. Insertion of a nasogastric tube yields clear aspirate. Her hematocrit is 28%. What is the most likely diagnosis?