Which of the following is not a constituent of mesorectum?
**Core Concept**
The mesorectum is a fibrofatty tissue enveloping the rectum, extending from the pelvic floor to the peritoneal reflection. It contains autonomic nerves, blood vessels, and lymphatic vessels that support the rectum.
**Why the Correct Answer is Right**
The correct answer is not a constituent of the mesorectum because it does not belong to the group of tissues that form this fibrofatty envelope. The mesorectum is primarily composed of **fat**, **connective tissue**, and **lymphatic vessels**. The correct answer, however, does not fit into this category.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the autonomic nerves are indeed a constituent of the mesorectum, facilitating the innervation of the rectum.
**Option B:** This option is incorrect because the blood vessels are a vital component of the mesorectum, supplying oxygen and nutrients to the rectum.
**Option C:** This option is incorrect because the lymphatic vessels are a crucial part of the mesorectum, responsible for draining lymph from the rectum.
**Clinical Pearl / High-Yield Fact**
The mesorectum is a key anatomical structure in the context of rectal cancer surgery, as it contains the primary lymphatic drainage of the rectum. A total mesorectal excision (TME) is a surgical technique that aims to remove the entire mesorectum along with the rectum to prevent cancer recurrence.
**Correct Answer:** D.