**Core Concept**
The **portal vein** is a vital vessel in the hepatic portal system, playing a crucial role in the digestive process by transporting blood from the gastrointestinal tract to the liver for detoxification and nutrient processing. The formation of the portal vein is essential for understanding hepatic circulation and the pathophysiology of various liver diseases.

**Why the Correct Answer is Right**
The portal vein is formed by the confluence of the **superior mesenteric vein** and the **splenic vein**, which occurs behind the neck of the pancreas. This junction marks the beginning of the portal vein, which then ascends to the liver, dividing into right and left branches that supply the respective lobes of the liver.

**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 is not related to the formation of the portal vein.
**Option C:** Incorrect because, although the splenic vein is involved, it alone does not form the portal vein.
**Option D:** Incorrect as it is not a correct description of the portal vein's formation.

**Clinical Pearl / High-Yield Fact**
The portal vein's formation is crucial for liver function, and any obstruction or thrombosis in this vein can lead to **portal hypertension**, characterized by splenomegaly, ascites, and varices. Understanding the anatomy of the portal vein is essential for diagnosing and managing conditions related to the liver and portal circulation.

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