**Core Concept:** The duodenum is the first part of the small intestine, divided into three parts: the head (pylorus), body, and tail (ampulla of Vater). The third part of the duodenum receives blood supply from the superior mesenteric artery, drains into the common hepatic duct and cystic duct, and is connected to the head of the pancreas.

**Why the Correct Answer is Right:** The correct answer, **C. Ampulla of Vater**, is the correct choice because it represents the endpoint of the third part of the duodenum. The ampulla of Vater is a crucial structure as it connects the third part of the duodenum to the common bile duct, which is a part of the biliary system. Additionally, it is a junction where the common hepatic duct from the liver and the cystic duct from the gallbladder merge to form the common bile duct.

**Why Each Wrong Option is Incorrect:**

**A. Pylorus:** The pylorus is the opening of the stomach into the duodenum and is not a part of the third part of the duodenum.

**B. Ampulla of Vater:** The ampulla of Vater is the correct answer, as mentioned above, but the choice **B** is incorrect because it refers to the junction between the third part of the duodenum and the biliary system, not the blood supply.

**D. Head of the Pancreas:** While the head of the pancreas is connected to the third part of the duodenum, the correct answer is **C** because it specifically addresses the blood supply to the third part of the duodenum.

**Clinical Pearl:** The duodenum and the ampulla of Vater are connected to the biliary system, which is crucial for bile production, storage, and transportation. This connection is essential for the digestion and absorption of fats and fat-soluble vitamins. The blood supply to the third part of the duodenum is critical for understanding the anatomy and physiology of this region, and it helps in diagnosing and treating gastrointestinal disorders.