**Core Concept**
The Gastrinoma triangle is an anatomical region in the pancreas that is commonly associated with the development of gastrinomas, a type of neuroendocrine tumor that secretes excess gastrin, leading to excessive acid production in the stomach. The triangle is defined by the confluence of the cystic and common bile ducts superiorly, the second and third portions of the duodenum inferiorly, and the neck of the pancreas medially.

**Why the Correct Answer is Right**
Gastrinomas typically occur in the head of the pancreas or the duodenum, within the Gastrinoma triangle. The excess gastrin produced by these tumors stimulates the parietal cells in the stomach to secrete more hydrochloric acid, leading to peptic ulcers and other complications. The Gastrinoma triangle is a useful landmark for surgeons and radiologists to identify and localize these tumors for treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the cystic and common bile ducts are indeed part of the superior border of the Gastrinoma triangle.
**Option B:** This option is incorrect because the second and third portions of the duodenum are part of the inferior border of the Gastrinoma triangle.
**Option C:** This option is incorrect because the neck of the pancreas is part of the medial border of the Gastrinoma triangle.

**Clinical Pearl / High-Yield Fact**
The Gastrinoma triangle is a classic example of an anatomical landmark that is clinically relevant in the diagnosis and treatment of gastrinomas. Remember that gastrinomas often occur in the head of the pancreas or the duodenum, and that the Gastrinoma triangle is a useful guide for localization and surgical resection.

**Correct Answer:** D