50 Year old man became dizzy while passing stool and noticed fresh blood in stool. Previous two concurrent colonoscopic examinations done for routine screening of carcinoma colon were normal. What is the most likely cause of bleed?

Disease Features Early carcinoma colon Cancer of right side of colon present with occult bleeding Left side with obstructive symptoms. Cancers arising in the rectosigmoid are often associated with hematochezia, tenesmus, and narrowing of the caliber of stool. Sigmoid diveiculitis Age : 59 years Abrupt in onset, usually painless, sometimes massive, and often from the right colon. Since Lesions are easily picked up on sigmoidoscopy, hence choice B is unlikely Microscopic colitis Presents with watery non-bloody diarrhea and hence ruled out. MC is comprised of two distinct histological forms -- collagenous colitis and lymphocytic colitis, both of which frequently occur in older populations. Dilatation of veins of colon Angiodysplasia is a degenerative lesion of previously healthy blood vessels found most commonly in the cecum and proximal ascending colon. Located in the cecum and ascending colon, 15% are located in the jejunum and ileum, and the remainder is distributed throughout the alimentary tract. These lesions typically are non-palpable and small (< 5 mm). Angiodysplasia is the most common vascular abnormality of the Gl tract. After diveiculosis, it is the second leading cause of lower Gl bleeding in patients older than 60 years.