**Core Concept**
The mesentery of the small intestine is a fold of tissue that attaches the small intestine to the posterior abdominal wall, providing a route for blood vessels and nerves to supply the intestine. The mesentery is a key anatomical landmark in the abdominal cavity, and its relationship to surrounding structures is crucial for understanding abdominal anatomy.

**Why the Correct Answer is Right**
The left gonadal vessels (testicular or ovarian arteries and veins) arise from the abdominal aorta below the level where the mesentery of the small intestine attaches to the posterior abdominal wall. In contrast, the right gonadal vessels cross the root of the mesentery, which is the part where the mesentery attaches to the posterior abdominal wall. This is why the left gonadal vessels are not crossed by the mesentery, whereas the right gonadal vessels are. The third part of the duodenum, the aorta, and the right ureter all cross the root of the mesentery, as they are located anterior to the root of the mesentery.

**Why Each Wrong Option is Incorrect**
**Option B:** The third part of the duodenum crosses the root of the mesentery, which is the part where the mesentery attaches to the posterior abdominal wall. This is why it is crossed by the mesentery.
**Option C:** The aorta is located anterior to the root of the mesentery and crosses it, as it supplies blood to the abdominal organs.
**Option D:** The right ureter crosses the root of the mesentery, which is the part where the mesentery attaches to the posterior abdominal wall.

**Clinical Pearl / High-Yield Fact**
It is essential to remember the relationship between the mesentery of the small intestine and surrounding structures, particularly the right gonadal vessels, which cross the root of the mesentery. This knowledge is crucial for understanding abdominal anatomy and planning surgical procedures.

**✓ Correct Answer: A. Left gonadal vessels**