**Core Concept**
A portocaval shunt is a surgical procedure aimed at relieving portal hypertension by diverting blood flow from the portal vein to the inferior vena cava. This is achieved by creating an anastomosis between a vein in the portal circulation and a vein in the systemic circulation.

**Why the Correct Answer is Right**
The correct answer involves identifying a suitable vein for anastomosis in the portal system. The superior mesenteric vein (SMV) and the splenic vein merge to form the portal vein. To create a portocaval shunt, a vein that drains into the portal circulation needs to be anastomosed to a vein in the systemic circulation. The superior mesenteric vein is a suitable option as it drains into the portal vein and can be anastomosed to a vein in the systemic circulation, such as the inferior vena cava.

**Why Each Wrong Option is Incorrect**
* **Option B:** The splenic vein is another potential option for a portocaval shunt, but it is not the most commonly used vein due to its smaller diameter and location.
* **Option C:** The portal vein itself cannot be anastomosed to the inferior vena cava as it would create a direct connection between the portal and systemic circulations, bypassing the liver.
* **Option D:** The hepatic veins are not suitable for a portocaval shunt as they drain blood from the liver into the inferior vena cava and do not participate in the portal circulation.

**Clinical Pearl / High-Yield Fact**
A portocaval shunt is typically performed in patients with severe portal hypertension and liver cirrhosis. However, it can lead to complications such as encephalopathy due to the diversion of blood flow around the liver.

**Correct Answer: C. Superior mesenteric vein.**