**Core Concept**
The hepatic portal vein is a vital vessel that plays a central role in the hepatic portal system, which is responsible for directing blood from the gastrointestinal tract and spleen to the liver for detoxification and nutrient processing. The **hepatic portal vein** is formed by the confluence of two major vessels that drain blood from the digestive organs. This convergence occurs outside the liver, specifically behind the neck of the pancreas.

**Why the Correct Answer is Right**
The correct answer involves the union of the **superior mesenteric vein** and the **splenic vein**. The **superior mesenteric vein** drains blood from the small intestine, while the **splenic vein** drains blood from the spleen and parts of the pancreas and stomach. These two veins merge to form the **hepatic portal vein**, which then enters the liver, distributing blood to the hepatic sinusoids for processing.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not accurately represent the primary vessels involved in forming the hepatic portal vein.
**Option B:** Incorrect as it is not the correct combination of vessels that merge to form the hepatic portal vein.
**Option C:** Incorrect because, although the splenic vein is involved, the other vessel mentioned is not the correct one.
**Option D:** Incorrect as it does not correctly identify the major vessels that unite to form the hepatic portal vein.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the hepatic portal vein is essential for the liver's function in metabolizing nutrients and detoxifying substances. Obstruction of the hepatic portal vein can lead to **portal hypertension**, characterized by increased pressure within the portal vein, which can cause severe complications such as varices and ascites.

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