**Core Concept**
The pancreas contains islet cells that secrete various hormones regulating glucose metabolism and blood sugar levels. These hormones include insulin, glucagon, somatostatin, pancreatic polypeptide, and amylin.

**Why the Correct Answer is Right**
Insulin is secreted by beta cells in the islets of Langerhans, promoting glucose uptake by cells and reducing blood glucose levels. Glucagon is secreted by alpha cells, increasing glucose release from stored glycogen and promoting gluconeogenesis. Somatostatin, pancreatic polypeptide, and amylin are secreted by delta cells, gamma cells, and beta cells, respectively, and play roles in regulating pancreatic secretion and insulin release. The correct answer is a hormone not secreted by the islet cells of the pancreas.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because insulin is indeed secreted by the beta cells in the islets of Langerhans.
**Option B:** This option is incorrect because glucagon is secreted by the alpha cells in the islets of Langerhans.
**Option C:** This option is incorrect because somatostatin is secreted by the delta cells in the islets of Langerhans.
**Option D:** This option is incorrect because pancreatic polypeptide is secreted by the PP cells in the islets of Langerhans.

**Clinical Pearl / High-Yield Fact**
The islet cells of the pancreas are responsible for regulating blood sugar levels through the secretion of various hormones. Damage to these cells can lead to diabetes mellitus, emphasizing the importance of preserving pancreatic function.

**Correct Answer: D. Growth hormone**