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?