A 65-year-old man consults a physician because he has had severe alternating constipation and diarrhea for the past two months. Hemoccult test on a stool sample is positive for occult blood. Colonoscopy demonstrates a large annular, encircling mass 25 cm above the anal verge, in the descending colon. To which of the following nodes would this tumor be most likely to first metastasize?

**Core Concept**
The question is testing the understanding of the lymphatic drainage of the colon, specifically the descending colon, and the likelihood of metastasis to regional lymph nodes. The lymphatic drainage of the colon follows the pattern of the blood supply, with the superior mesenteric artery and its branches playing a crucial role.

**Why the Correct Answer is Right**
The descending colon receives its blood supply from the left colic artery, a branch of the inferior mesenteric artery. The lymphatic drainage of the descending colon follows the left colic artery and drains into the left colic lymph nodes. These nodes are located along the left colic artery and are part of the superior mesenteric lymph nodes. The tumor described in the question would most likely first metastasize to the left colic lymph nodes due to their proximity to the tumor site.

**Why Each Wrong Option is Incorrect**
**Option A:** Internal iliac lymph nodes are not the primary site for metastasis from the descending colon, as they receive lymph from the rectum and sigmoid colon.
**Option B:** Para-aortic lymph nodes are involved in the drainage of the small intestine and are not the primary site for metastasis from the descending colon.
**Option C:** Right colic lymph nodes are involved in the drainage of the right side of the colon and are not the primary site for metastasis from the descending colon.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the lymphatic drainage of the colon follows the pattern of the blood supply, and knowing the vascular supply of the colon is crucial for understanding its lymphatic drainage.

**Correct Answer:** . Left colic lymph nodes.

✓ Correct Answer: B. Inferior mesenteric