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What is Massive bleeding per rectum in a 70 yr old patient is due to - and explain in detail?
Causes of lower GI bleed in adults 


Local anorectal diseases - hemorrhoids (most common), anal fissures.
Diverticulosis (2nd MC)
Vascular ectasias, angiodysplasias
Neoplasm (polyps & adenocarcinoma)
Colitis (Ischemic, infectious, & IBD)


Although hemorrhoids are the MC cause of bleeding per rectum, it causes small bleed.
Diverticulosis is the MC cause of massive bleeding per rectum.
Diverticular bleeding is abrupt in onset usually painless, sometimes massive, and often from the right colon. The bleeding stops spontaneously in approx. 80% of patients and rebleeds in 20-25% of pts.