**Core Concept**
Gastric-secreting islet cell tumors, also known as gastrinomas, are a type of neuroendocrine tumor that secretes excessive amounts of gastrin, leading to peptic ulcers and diarrhea. These tumors are often associated with the Zollinger-Ellison syndrome.

**Why the Correct Answer is Right**
The most common hormone secreted by gastrinomas, excluding gastrin, is vasoactive intestinal polypeptide (VIP). VIPomas are a type of gastrinoma that secretes excessive amounts of VIP, leading to a syndrome characterized by severe diarrhea, hypokalemia, and achlorhydria. The mechanism involves the overproduction of VIP, which is a potent vasodilator and secretagogue that stimulates the release of chloride and bicarbonate ions into the intestinal lumen, leading to secretory diarrhea.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a hormone secreted by gastrinomas.

**Option B:** This option is incorrect because it is not the most common hormone secreted by gastrinomas, excluding gastrin.

**Option D:** This option is incorrect because it is not a hormone secreted by gastrinomas.

**Clinical Pearl / High-Yield Fact**
Gastrinomas are often difficult to diagnose due to their rarity and nonspecific symptoms. A high index of suspicion and the use of imaging studies, such as CT scans and endoscopic ultrasonography, are essential in diagnosing these tumors.

**Correct Answer: C. Vasoactive intestinal polypeptide (VIP).**