Which of the following procedures involves the evaluation of the most distal portions of the gastrointestinal tract?

The procedure that involves the evaluation of the most distal portions of the gastrointestinal tract is flexible sigmoidoscopy. This technique specifically examines the sigmoid colon and the rectum, which are the terminal parts of the large intestine. During the procedure, a flexible tube with a camera is inserted into the rectum, allowing for direct visualization and assessment of these areas.

In contrast, colonoscopy encompasses a broader examination of the entire colon, including regions proximal to the sigmoid colon. Endoscopy generally refers to visualizing the upper gastrointestinal tract, which includes the esophagus, stomach, and the first part of the small intestine but does not focus on the distal portions of the GI tract. An upper GI series is a radiographic examination of the upper gastrointestinal tract and does not provide direct visualization of the colon. Thus, flexible sigmoidoscopy is the most appropriate choice for evaluating the distal parts of the gastrointestinal tract, particularly the sigmoid and rectal regions.