B. Diverticulosis: This is the most common cause of lower gastrointestinal bleeding in adults over 60 years old. The patient's age and history of chronic constipation, which can increase intraluminal pressure and contribute to diverticula formation, support this diagnosis.

A. Angiodysplasia: This condition is more common in the elderly and can cause significant lower GI bleeding. The patient's age makes this a plausible diagnosis.
C. Colonic cancer: Although less common than diverticulosis as a cause of lower GI bleeding, colonic cancer is still a significant consideration, especially given the patient's age, which puts him in a higher risk category for colorectal cancer.

D. Peptic ulcer disease: Although more commonly associated with upper GI bleeding, peptic ulcers can occasionally cause lower GI bleeding, especially if the ulcer is located in the duodenum or stomach and erodes into a nearby structure. The patient's use of Clodrel (an antiplatelet agent) for ischemic heart disease increases the risk of bleeding from a peptic ulcer.
Ischemic colitis or other vascular events: Given the patient's history of ischemic heart disease and use of antiplatelet therapy, ischemic events affecting the bowel could lead to bleeding and should not be overlooked.

Meckel's diverticulum: A congenital anomaly that can cause lower GI bleeding, usually in younger patients but can be considered in the differential diagnosis of any age group if common causes are ruled out.
Inflammatory bowel disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause lower GI bleeding but are less common in new presentations at this age without prior symptoms.
Hemorrhoids or anal fissures: While common causes of lower GI bleeding, they typically present with smaller amounts of blood and are less likely to cause "massive" bleeding as described in the scenario.