**Question:** Portal vein formed by the union of-

A. superior mesenteric vein and inferior mesenteric vein
B. splenic vein and superior mesenteric vein
C. superior mesenteric vein and inferior mesenteric vein
D. left gastric vein and right gastric vein

**Correct Answer:** C. superior mesenteric vein and inferior mesenteric vein

**Core Concept:**
The portal vein is a major vein in the human body that carries deoxygenated blood enriched with nutrients from the digestive system to the liver. It plays a crucial role in supplying oxygen and nutrients to the liver, spleen, and pancreas. The portal vein is formed by the union of several smaller veins.

**Why the Correct Answer is Right:**
The portal vein is formed by the union of two major veins: the superior mesenteric vein (coming from the small intestine) and the inferior mesenteric vein (coming from the large intestine). This combination ensures that the portal vein receives blood from both the upper and lower parts of the gastrointestinal tract, ensuring efficient delivery of nutrients to the liver, spleen, and pancreas.

**Why Each Wrong Option is Incorrect:**
A. The superior mesenteric vein and inferior mesenteric vein are incorrect because they do not form the portal vein. Instead, these veins combine to form the superior and inferior mesenteric veins, which ultimately empty into the superior and inferior vena cava, respectively.

B. This option is incorrect because the splenic vein is responsible for carrying blood from the spleen, not the formation of the portal vein. The splenic vein joins the portal vein after it has formed.

D. This option is incorrect as the left gastric vein and right gastric vein are small veins that drain the stomach wall and do not contribute to the formation of the portal vein.

**Clinical Pearl:**
Understanding the correct formation of the portal vein is essential for medical students and physicians, as it helps to appreciate the anatomy and physiology of blood flow in the gastrointestinal system. This knowledge is crucial for diagnosing and treating liver diseases, gastrointestinal disorders, and understanding the consequences of variceal bleeding. In-depth knowledge of portal vein formation can also assist in recognizing clinical signs and symptoms related to portal hypertension, such as splenomegaly, ascites, and varicosities.