**Core Concept**
Multiple Endocrine Neoplasia Type 1 (MEN 1) is a hereditary disorder characterized by the occurrence of tumors in multiple endocrine glands. MEN 1 is caused by mutations in the MEN1 gene, leading to the development of neuroendocrine tumors (NETs) in the parathyroid glands, pituitary gland, and pancreas.

**Why the Correct Answer is Right**
The most common neuroendocrine tumor in MEN 1 is a gastrinoma, which is a type of pancreatic NET that secretes gastrin, leading to excessive gastric acid production. Gastrinomas are usually located in the duodenum and are often associated with Zollinger-Ellison syndrome. The high gastrin levels stimulate the parietal cells in the stomach to produce excessive gastric acid, causing peptic ulcers and diarrhea.

**Why Each Wrong Option is Incorrect**
* **Option A:** Insulinomas are the most common type of pancreatic NET, but they are not the most common in MEN 1.
* **Option B:** Somatostatinomas are rare NETs that secrete somatostatin, but they are not the most common in MEN 1.
* **Option D:** VIPomas are rare NETs that secrete vasoactive intestinal peptide (VIP), but they are not the most common in MEN 1.

**Clinical Pearl / High-Yield Fact**
Gastrinomas in MEN 1 are often resistant to traditional medical therapy, and surgical resection of the tumor is usually required to control symptoms.

**Correct Answer: C. Gastrinoma**