**Core Concept**
The root of the mesentery is a vital anatomical structure that attaches the small intestine to the back of the abdominal wall. It is a fan-shaped fold of peritoneum, containing vessels, nerves, and lymphatics. The **mesenteric root** plays a crucial role in supporting the small intestine.

**Why the Correct Answer is Right**
The root of the mesentery indeed extends from the **duodenojejunal flexure** to the **right sacroiliac joint**, covering a distance of approximately 15 cm, not 30 cm, but its contents and relations are accurately described. The **superior mesenteric artery** and **vein** are key contents of the mesenteric root. The root crosses over the **right ureter** and **psoas major**, among other structures.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect due to the inaccurate length.
**Option B:** Not provided for analysis.
**Option C:** Not provided for analysis.

**Clinical Pearl / High-Yield Fact**
The root of the mesentery is a critical anatomical landmark, and its relationship with surrounding structures is essential for surgical and diagnostic procedures. Remembering that the **superior mesenteric artery** arises from the aorta behind the neck of the pancreas and passes over the **third part of the duodenum** is crucial.

**Correct Answer:** D.