The commonest cause of significant lower gastrointestinal bleed in a middle aged person with unknown reason is

.Diveiculosis is the initial primary stage of the disease, wherein there is hyperophy, muscular incoordination leading to increased segmentation and increased intraluminal pressure. At this stage they are asymptomatic, but often get severe spasmodic pain due to colonic segmentation called as painful diveicular disease. Sigmoid diveicula causing pericolic abscess as a known complication. Resection and primary anastomosis can be done in sigmoid diveicula after proper bowel preparation electively. ref:SRB&;s manual of surgery,ed 3,pg no 822.