**Core Concept**
Zollinger-Ellison syndrome (ZES) is a rare disorder characterized by excessive gastric acid secretion, leading to severe peptic ulcers and diarrhea. This condition is primarily caused by a gastrin-producing tumor, usually a gastrinoma, which stimulates the parietal cells of the stomach to secrete more acid.

**Why the Correct Answer is Right**
The gastrinoma in ZES secretes excessive amounts of gastrin, a hormone that stimulates the parietal cells of the stomach to secrete gastric acid. The increased gastrin levels lead to hypergastrinemia, resulting in an overproduction of gastric acid and the characteristic symptoms of ZES. The parietal cells are stimulated by gastrin, which binds to the gastrin receptors on the surface of these cells, triggering an increase in acid secretion.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastrinomas are the primary cause of ZES, so this option is incorrect.
**Option B:** While not the primary cause, other tumors like pancreatic neuroendocrine tumors can also lead to ZES by secreting gastrin or other substances that stimulate acid secretion.
**Option C:** Gastric carcinoids can also produce gastrin and lead to ZES, so this option is incorrect.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that ZES can be caused by various tumors, not just gastrinomas, and that the diagnosis often requires a combination of clinical presentation, laboratory tests, and imaging studies to identify the underlying cause.

**Correct Answer:** D. Gastric carcinoids are not the primary cause of ZES.