**Core Concept**
The portal venous system is a network of blood vessels that carries blood from the gastrointestinal tract, spleen, and pancreas to the liver for detoxification and nutrient processing. The presence of valves in the portal venous system is crucial for preventing blood reflux and ensuring proper blood flow to the liver.

**Why the Correct Answer is Right**
Valves are present at the hepatic portal vein and its tributaries, particularly at the confluence of the superior mesenteric vein and the splenic vein. These valves prevent blood from flowing back into the gastrointestinal tract and ensure that blood is directed towards the liver for processing. The hepatic portal vein is a major vessel that carries blood from the gastrointestinal tract to the liver, and the presence of valves at its confluence is essential for maintaining proper blood flow.

**Why Each Wrong Option is Incorrect**
* **Option A:** The hepatic portal vein is not a valve itself, but rather a vessel that carries blood from the gastrointestinal tract to the liver.
* **Option B:** The inferior vena cava is a major vein that carries blood from the lower body to the heart, but it does not have valves in the portal venous system.
* **Option D:** The hepatic veins are responsible for carrying blood from the liver to the inferior vena cava, but they do not have valves in the portal venous system.

**Clinical Pearl / High-Yield Fact**
The presence of valves in the portal venous system is essential for maintaining proper blood flow to the liver and preventing blood reflux. This is particularly important in conditions such as portal hypertension, where the presence of valves can help to prevent blood from flowing back into the gastrointestinal tract.

**Correct Answer: C. The confluence of the superior mesenteric vein and the splenic vein.**