**Core Concept:**
The gastrointestinal (GI) tract is supplied by multiple blood vessels and nerves, with some regions receiving blood supply from specific sources. The colon is a part of the large intestine, which plays a crucial role in the absorption of water and electrolytes from the remaining fecal matter before it is expelled from the body.

**Why the Correct Answer is Right:**
The colon receives its blood supply mainly from the superior rectal artery and the inferior mesenteric artery, both branches of the internal iliac artery (a.k.a. the terminal portion of the abdominal aorta). The superior rectal artery supplies the rectum and sigmoid colon, while the inferior mesenteric artery supplies the ascending, transverse, and descending colon.

**Why Each Wrong Option is Incorrect:**
A. The superior mesenteric artery supplies the small intestine and part of the colon, making it incorrect for the entire colon.
B. The superior mesenteric artery is incorrect because it supplies the small intestine and part of the colon, not the entire colon.
C. The superior rectal artery supplies the rectum and sigmoid colon, not the entire colon.
D. The external iliac artery is incorrect as it supplies the pelvic organs and does not directly contribute to the colon's blood supply.

**Why the Correct Answer is Right:**
The correct answer, D, is excluded since the superior rectal artery and inferior mesenteric artery supply the colon, while the external iliac artery supplies the pelvic organs. The superior mesenteric artery and superior mesenteric vein form the superior mesenteric artery and vein, which are not related to the colon's blood supply.

**Why Each Wrong Option is Incorrect:**
A, B, and C are incorrect because they involve arteries supplying specific segments of the colon (superior mesenteric artery, superior rectal artery, and inferior mesenteric artery, respectively) rather than the entire colon.

**Clinical Pearl:**
Sigmoidoscopy is a common diagnostic procedure that involves inserting a flexible tube with a light source and camera into the rectum and sigmoid colon, which can help detect abnormalities such as polyps, cancer, and inflammatory bowel disease (IBD). This examination aids in assessing the lower gastrointestinal tract and can help guide treatment decisions or further investigations, such as colonoscopy or imaging studies.