What is the most appropriate next investigation for a patient with diarrhea, blood in stool, and a high number of daily bowel movements?

In this scenario, the patient presents with diarrhea that is bloody and a high frequency of daily bowel movements, which raises significant concern for a lower gastrointestinal tract pathology, possibly inflammatory or infectious in nature. Colonoscopy is the most appropriate next step because it allows for direct visualization of the colon and the ability to sample biopsies from potential lesions or areas of inflammation.

Colonoscopy is particularly valuable in diagnosing conditions such as ulcerative colitis, Crohn's disease, and colorectal cancer, all of which can present with the symptoms described. This investigation not only helps in confirming the diagnosis but also enables therapeutic interventions, such as polypectomy or treatment of bleeding sites.

Other imaging modalities, such as a CT scan of the abdomen, may provide useful information about the abdominal organs but do not allow for direct examination or intervention within the colon itself. A plain X-ray of the abdomen is typically not helpful for this clinical scenario, as it doesn't provide detailed information about the mucosal surface or identify lesions. Rigid sigmoidoscopy, while it can visualize the rectum and lower part of the sigmoid colon, does not allow for examination of the entire colon, limiting its effectiveness in case of widespread disease or pathology.

In summary, a colonoscopy is the most