**Core Concept**
The portal vein is a vital blood vessel that plays a crucial role in the hepatic portal system, responsible for transporting nutrient-rich blood from the gastrointestinal tract to the liver for detoxification, metabolism, and distribution to the rest of the body.

**Why the Correct Answer is Right**
The portal vein is formed by the confluence of the superior mesenteric vein and the splenic vein. This union occurs in the hepatoduodenal ligament, near the liver's porta hepatis. The superior mesenteric vein drains the small intestine and the right half of the colon, while the splenic vein drains the spleen, the pancreas, and the left half of the colon. The combined portal vein then flows into the liver, where it branches into smaller portal veins that eventually drain into the hepatic sinusoids.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the inferior mesenteric vein, which drains the left half of the colon, typically empties into the splenic vein, not forming the portal vein.
**Option B:** This option is incorrect because the hepatic portal vein is not formed by the union of the inferior mesenteric vein and the splenic vein.
**Option C:** This option is incorrect because the superior mesenteric vein alone does not form the portal vein; it must combine with the splenic vein.

**Clinical Pearl / High-Yield Fact**
The portal vein's unique anatomy can be remembered using the mnemonic "SSM" for Splenic, Superior Mesenteric, and the combined portal vein. This helps students recall the correct answer and the key vessels involved in the hepatic portal system.

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