**Core Concept**
Somatostatinoma is a rare neuroendocrine tumor that secretes the hormone somatostatin. This hormone has a wide range of effects on the body, including inhibition of the release of other hormones such as gastrin, secretin, and cholecystokinin (CCK). The association between somatostatinoma and gallstones lies in the inhibition of CCK, which normally stimulates the release of pancreatic enzymes and bile salts.

**Why the Correct Answer is Right**
Somatostatin inhibits CCK, leading to decreased bile salt secretion and increased bile cholesterol concentration. This results in supersaturation of bile with cholesterol, promoting the formation of cholesterol gallstones. Additionally, somatostatin inhibits gastrin, leading to decreased gastric acid secretion and potentially increasing the pH of the gallbladder bile, which can also contribute to gallstone formation.

**Why Each Wrong Option is Incorrect**
**Option A:** Insulinoma is a tumor that secretes excess insulin, leading to hypoglycemia. While it has various systemic effects, it is not directly associated with gallstone formation.

**Option B:** VIPoma is a tumor that secretes vasoactive intestinal peptide (VIP), leading to a syndrome characterized by diarrhea, hypokalemia, and achlorhydria. It does not have a direct association with gallstone formation.

**Option D:** Glucagonoma is a tumor that secretes excess glucagon, leading to hyperglycemia, weight loss, and a characteristic rash. Like the other options, it does not have a direct association with gallstone formation.

**Clinical Pearl / High-Yield Fact**
Somatostatinoma is a rare tumor that can present with a wide range of systemic effects, including diabetes mellitus, gallstones, and steatorrhea. It is often associated with multiple endocrine neoplasia type 1 (MEN1) syndrome.

**✓ Correct Answer: C. Somatostatinoma**