## **Core Concept**
The duodenum is divided into four parts: the first (D1) or superior part, the second (D2) or descending part, the third (D3) or horizontal part, and the fourth (D4) or ascending part. The third part of the duodenum is significant because of its relationship with nearby structures, particularly the major vessels and the root of the mesentery.

## **Why the Correct Answer is Right**
The third part of the duodenum is crossed anteriorly by the **superior mesenteric vessels** (the superior mesenteric artery and vein). These vessels arise from the aorta and the splenic vein, respectively, and pass anterior to the third part of the duodenum to supply the intestine from the distal duodenum to the proximal two-thirds of the transverse colon. This relationship is clinically significant because compression of the duodenum by these vessels can lead to a condition known as superior mesenteric artery syndrome.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The portal vein does not directly cross the third part of the duodenum; it is formed behind the neck of the pancreas by the confluence of the superior mesenteric vein and the splenic vein.
- **Option B:** The splenic vein primarily runs behind the pancreas and does not cross the third part of the duodenum; it joins the superior mesenteric vein to form the portal vein.
- **Option D:** The gastroduodenal artery, a branch of the common hepatic artery, primarily supplies the duodenum and pancreas but does not characteristically cross the third part of the duodenum.

## **Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that the third part of the duodenum can be compressed by the superior mesenteric vessels, particularly in cases of severe weight loss or certain body habitus, leading to **superior mesenteric artery syndrome**. This condition presents with symptoms of bowel obstruction.

## **Correct Answer:** C. Superior mesenteric vessels.