60 yr old man comes to ER complaining of bright red blood per rectum. Bleeding began abruptly several hours ago. Has orthostatic hypotension but no abdominal pain, cramps, fever, nausea, or vomiting. No previous hx of bleeding or ab pain, but has a hx of CAD and takes aspirin as a "blood thinner." He is afebrile, slightly hypotensive, and tachycardic, but stable. On exam, decreased skin turgor and dry mucous membranes are noted. Ne has no abdominal tenderness. Rectal exam is positive for gross blood. What is the most likely diagnosis: A) Arteriovenous malformation b) diverticulitis C) infectious colitis d) ischemic colitis E) Ulcerative colitis.