**Core Concept**
Multiple Endocrine Neoplasia Type I (MEN I) is a genetic disorder characterized by the development of tumors in the parathyroid glands, pancreas, and anterior pituitary. Among these, gastrinoma is the most frequently occurring tumor, leading to Zollinger-Ellison syndrome due to excessive gastric acid secretion.

**Why the Correct Answer is Right**
In MEN I, gastrinoma arises in the pancreas and is the most common endocrine tumor, occurring in approximately 30–50% of cases. These tumors secrete gastrin, which stimulates the parietal cells in the stomach to produce excessive hydrochloric acid. This leads to peptic ulcers and hypergastrinemia. The tumor is typically located in the duodenal or pancreatic head region and is associated with a strong genetic predisposition via mutations in the MEN1 gene.

**Why Each Wrong Option is Incorrect**
Option A: Insulinoma is more commonly associated with insulinoma syndrome (e.g., hypoglycemia), but it is not the most common tumor in MEN I. It is more frequent in sporadic cases or in insulinoma syndromes, not MEN I.
Option C: Glucagonoma is rare in MEN I and is more commonly seen in sporadic cases or in other syndromes like insulinoma or glucagonoma syndromes.
Option D: Somatotrophinoma (which produces growth hormone) is not a hallmark of MEN I; it is more commonly linked to acromegaly and is typically seen in sporadic cases or in other endocrine disorders.

**Clinical Pearl / High-Yield Fact**
In MEN I, **gastrinoma** is the most common endocrine tumor and is a key diagnostic clue. Remember: **"MEN I = Parathyroid + Pancreatic (gastrinoma) + Pituitary"** — with gastrinoma being the most frequent pancreatic tumor.

✓ Correct Answer: B. Gastrinoma