**Core Concept**
The portal vein is a vital blood vessel that plays a crucial role in the hepatic circulation, carrying deoxygenated blood from the gastrointestinal tract to the liver for detoxification and nutrient processing. The portal vein is formed by the confluence of the superior mesenteric vein and the splenic vein.

**Why the Correct Answer is Right**
The correct answer is related to the anatomy and formation of the portal vein. The portal vein is formed by the confluence of the superior mesenteric vein and the splenic vein. This is a critical concept in understanding the hepatic circulation and the pathophysiology of portal hypertension. The superior mesenteric vein carries blood from the small intestine, while the splenic vein carries blood from the spleen and the left half of the stomach.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the hepatic portal vein is not formed by the confluence of the azygos vein and the inferior vena cava. The azygos vein is a major vein that drains blood from the thoracic wall and returns it to the inferior vena cava.
**Option B:** This option is incorrect because the portal vein is not formed by the confluence of the renal veins. The renal veins drain blood from the kidneys and return it to the inferior vena cava.
**Option C:** This option is incorrect because the portal vein is not formed by the confluence of the inferior mesenteric vein and the superior mesenteric vein only. The portal vein is formed by the confluence of the superior mesenteric vein and the splenic vein.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the portal vein is a high-pressure system due to the resistance in the liver, and any obstruction or thrombosis in the portal vein can lead to severe consequences, including portal hypertension and liver failure.

**Correct Answer: C.