Question: An 84-year-old man presented to the emergency department with 1 episode of large volume rectal bleed. He has a medical history that includes paroxysmal atrial fibrillation, diabetes mellitus, ischemic heart disease, and hypertension for which he takes metformin, insulin, digoxin, pantoprazole, simvastatin, aspirin, and betamin. On initial examination, he was afebrile with a systolic blood pressure (SBP) of 110, pulse rate of 110 and had a SBP postural decrease of 30 mmHg. Gastrointestinal examination revealed generalized pallor, dry mucous membranes, and a soft abdomen with no tenderness or masses on palpation. There was fresh blood and clots on rectal examination. Fluid resuscitation was initiated and his initial hemoglobin returned at 66 g/L with an International Normalized Ratio of 1.09. Two units of pack cells were transfused and a CT mesenteric angiogram was organized. What are the likely causes of a torrential lower gastrointestinal bleed?