**Core Concept**
The patient's presentation with lethargy, yellow urine, and pruritus is suggestive of obstructive jaundice, which is caused by a tumor located in the head of the pancreas. The tumor is obstructing the bile duct, leading to a buildup of bilirubin in the blood, causing jaundice, and pruritus.

**Why the Correct Answer is Right**
The head of the pancreas is located near the bile duct, and a tumor in this region can easily obstruct the bile duct, leading to the patient's symptoms. The bile duct is a common pathway for bile to drain from the liver and gallbladder into the small intestine. Obstruction of this duct can cause a buildup of bilirubin, which can lead to jaundice, yellow urine, and pruritus.

**Why Each Wrong Option is Incorrect**
**Option A:** The body and tail of the pancreas are not typically associated with bile duct obstruction.
**Option B:** The neck of the pancreas is a transitional zone between the head and body, but it is not the most common location for a tumor to obstruct the bile duct.
**Option C:** The duodenum is a part of the small intestine, and while it is near the head of the pancreas, a tumor in this region would not typically obstruct the bile duct.

**Clinical Pearl / High-Yield Fact**
A tumor in the head of the pancreas can also cause weight loss, fatigue, and abdominal pain due to the tumor itself and the subsequent obstruction of the bile duct.

**Correct Answer: C. The head of the pancreas.**