Which procedure requires the use of a flexible endoscope?

The procedure that involves the use of a flexible endoscope is colonoscopy. This technique allows a direct view of the interior lining of the colon and the rectum, enabling the examination for abnormalities such as polyps, tumors, or signs of inflammatory bowel disease.

Colonoscopy is particularly advantageous because of the flexibility of the endoscope, which can navigate the curves and angles of the colon. The flexible endoscope is equipped with a light and camera, as well as access ports that enable the physician to perform biopsies or remove polyps during the procedure, making it a key tool in both diagnosis and therapeutic intervention in gastrointestinal disturbances.

Other procedures listed, such as esophagoscopy, cystoscopy, and gastroscopy, utilize flexible or rigid endoscopes depending on the specific site being examined. However, among the choices provided, colonoscopy distinctly emphasizes the use of flexible endoscopy specifically designed for examining the colonic structure and is thus the best answer in this context.