<s>[INST] context: Painless hematochezia or bright red lower GI bleeding can come from many sources. While bright red lower GI bleeding tends to indicate lower GI bleeding (bleeding distal to the ligament of Treitz), brisk upper GI bleeding can also be the source. The clinical manifestations of such bleeding range from negligible to hemodynamic instability, depending upon the rate of bleeding. The differential diagnosis for painless hematochezia includes AV malformations, gastric erosions, esophageal varices, esophagitis, duodenal or gastric ulcer, hemorrhoids, diveiculosis, and colonic neoplasm. Diveiculitis occurs when a colonic outpouching or diveiculum becomes inflamed. Patients tend to be elderly and present with fever, abdominal pain, and abdominal tenderness on examination. While painful, these lesions do not bleed significantly (unlike their uninflamed counterpas in diveiculosis). Infectious colitis may present as rectal bleeding, but this bleeding is typically accompanied by pain, cramping, and fever. Causative organisms may include Salmonella, Shigella, Campylobacter jejuni, E. coli, and Entamoeba histolytica. Ischemic colitis may have rectal bleeding, but the hallmark of ischemic colitis is severe abdominal pain out of propoion to examination findings. [/INST] question: A 60-year-old comes to the casualty complaining of bright red blood per rectum. The bleeding began abruptly several hours prior to his visit. He has lightheadedness when he stands up rapidly, but has no abdominal pain, cramping, fever, nausea, or vomiting. He has no history of previous episodes of bleeding or abdominal pain, but has a history of coronary aery disease and takes aspirin as a "blood thinner". He is afebrile, slightly hypotensive and tachycardic, but stable. On examination, he has decreased skin turgor, and dry mucous membranes. He has no abdominal tenderness. Rectal examination is positive for gross blood. Which of the following is the most likely diagnosis? | options: A: Aeriovenous malformation B: Diveiculitis C: Infectious colitis D: Ischemic colitis | answer: A </s>