**Core Concept**
The formation of the portal vein is a crucial aspect of hepatic and gastrointestinal physiology, involving the convergence of veins that drain blood from the digestive organs. The **portal venous system** plays a key role in the delivery of nutrients to the liver. This process is essential for the liver's function in metabolism, detoxification, and production of biochemistry necessary for digestion.

**Why the Correct Answer is Right**
The correct answer involves the junction of the **superior mesenteric vein** and the **splenic vein**, which occurs behind the neck of the pancreas. This convergence creates the **portal vein**, a vital vessel that directs blood from the stomach, small intestine, and spleen to the liver. The **hepatic portal vein** then branches into the liver, distributing the blood throughout the hepatic sinusoids.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not accurately describe the formation of the portal vein.
**Option B:** Incorrect as it inaccurately represents the veins involved in forming the portal vein.
**Option C:** Incorrect because the formation of the portal vein does not involve the inferior mesenteric vein directly.

**Clinical Pearl / High-Yield Fact**
The portal vein's formation and function are critical in clinical settings, especially in conditions like portal hypertension, where increased pressure in the portal vein can lead to severe complications. Understanding the anatomy of the portal venous system is essential for diagnosing and managing such conditions.

**Correct Answer:** D. By the confluence of the superior mesenteric vein and the splenic vein.