**Core Concept**
The portal vein is a vital structure in the hepatic portal system, responsible for transporting blood from the gastrointestinal tract, spleen, and pancreas to the liver for detoxification and nutrient processing. The formation of the portal vein is a critical step in this process.

**Why the Correct Answer is Right**
The portal vein is formed by the union of the superior mesenteric vein and the splenic vein. This occurs at the confluence of these two veins, typically located at the neck of the pancreas. The superior mesenteric vein carries blood from the small intestine, while the splenic vein carries blood from the spleen and the stomach. The resulting portal vein then directs this blood towards the liver for further processing. The liver plays a crucial role in detoxifying the blood, metabolizing nutrients, and regulating various metabolic processes.

**Why Each Wrong Option is Incorrect**
* **Option A:** The superior mesenteric artery is a major artery that supplies blood to the small intestine, but it is not involved in the formation of the portal vein.
* **Option B:** The hepatic artery proper is a branch of the common hepatic artery that supplies oxygenated blood to the liver, but it is not involved in the formation of the portal vein.
* **Option D:** The inferior mesenteric vein is a smaller vein that drains the large intestine, but it is not involved in the formation of the portal vein.

**Clinical Pearl / High-Yield Fact**
The portal vein is prone to thrombosis, particularly in patients with cirrhosis or portal hypertension. This can lead to significant morbidity and mortality, highlighting the importance of early diagnosis and management of portal vein thrombosis.

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