What investigation should be performed first for a patient with bright red rectal bleeding?

For a patient experiencing bright red rectal bleeding, the most appropriate initial investigation is a flexible sigmoidoscopy. This procedure allows direct visualization of the lower part of the colon, including the rectum and sigmoid colon, which are common sites for hemorrhaging that presents as bright red blood.

Flexible sigmoidoscopy is advantageous because it is minimally invasive, can be performed quickly, and provides immediate diagnostic information. It also allows for therapeutic interventions, such as cauterization of lesions or polyps, if present. Since the bleeding is noted as bright red, it suggests an origin in the lower gastrointestinal tract, making flexible sigmoidoscopy particularly suitable for this case.

Other imaging or investigative techniques such as MRI of the abdomen, rigid sigmoidoscopy, or CT colonography, while they may be useful in certain scenarios, are not the first-line approaches for immediate evaluation in this context. MRI, for instance, is not primarily used for gastrointestinal bleeding, and CT colonography typically serves as a non-invasive alternative to a colonoscopy and may not provide the same level of immediate diagnostic clarity for active bleeding. Rigid sigmoidoscopy is limited to examination of the anal canal and lower rectum, thus would not assess the potential source