## **Core Concept**
The portal vein is a vital structure in the abdominal cavity that plays a crucial role in the hepatic portal system. It is formed by the confluence of two main veins that drain blood from the gastrointestinal tract, spleen, and pancreas. The hepatic portal system is essential for nutrient absorption and metabolism.

## **Why the Correct Answer is Right**
The correct answer, **superior mesenteric vein and splenic vein**, is accurate because the portal vein is indeed formed by the union of these two veins. The superior mesenteric vein drains blood from the small intestine, cecum, ascending colon, and transverse colon, while the splenic vein drains blood from the spleen, stomach (via short gastric veins), and pancreas (via pancreatic veins). Their junction occurs behind the neck of the pancreas and in front of the third part of the duodenum, forming the hepatic portal vein that carries blood to the liver for processing.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The inferior mesenteric vein and superior mesenteric vein do not directly form the portal vein. The inferior mesenteric vein usually drains into the splenic vein or directly into the portal vein, but it is not one of the two main tributaries.
- **Option B:** The common hepatic vein and splenic vein are not correct. The common hepatic vein drains into the inferior vena cava, not contributing to the formation of the portal vein.
- **Option C:** The cystic vein and superior mesenteric vein are not the primary tributaries. The cystic vein drains into the portal vein or the right hepatic vein but is not one of the main veins forming it.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the portal vein is formed behind the neck of the pancreas. This is clinically significant because it is a common site for pancreatic pathology, and understanding the anatomy helps in the diagnosis and management of related conditions. Additionally, the formation of the portal vein from the superior mesenteric and splenic veins highlights the unique hepatic portal system, which allows for the liver to first-pass metabolize nutrients absorbed from the gastrointestinal tract.

## **Correct Answer:** D. Superior mesenteric vein and splenic vein.