<prompt> The most useful investigation for profuse lower gastrointestinal bleeding isProctosigmoidoscopyColonoscopyDouble contrast barium enemaSelective aeriolgraphy  <response>: .Indications for colonoscopy includes Bleeding per rectum, resistant anaemia. To take biopsies from different pas of the bowel. To identify synchronous growths, ulcerative colitis. To remove polyps. When barium enema shows irregularity. For therapy--colonoscopic polypectomy, dilatation of stricture colon, fulgaration. Contraindicated in Acute ulcerative colitis. ref:SRB;s manual of surgery,ed 3,pg no 891 