**Core Concept**
Parietal cells in the stomach secrete a protein essential for the absorption of vitamin B12 by the ileum, highlighting the importance of the stomach in vitamin B12 metabolism. This protein plays a crucial role in the enterohepatic circulation of vitamin B12. The stomach's role in vitamin B12 absorption is a critical aspect of gastrointestinal physiology.

**Why the Correct Answer is Right**
The protein secreted by parietal cells is intrinsic factor, a glycoprotein that binds to vitamin B12 in the duodenum. This binding is essential for the subsequent absorption of vitamin B12 by the ileum. Intrinsic factor binds to vitamin B12 in the stomach, forming a complex that resists degradation in the acidic environment. This complex is then absorbed by the ileal mucosa, where vitamin B12 is released and subsequently transported to the liver for storage. The intrinsic factor-vitamin B12 complex is a critical step in the enterohepatic circulation of vitamin B12.

**Why Each Wrong Option is Incorrect**
**Option B:** Gastrin is a hormone secreted by G cells in the stomach that stimulates the secretion of gastric acid and pepsinogen by parietal cells. While gastrin is involved in gastric secretion, it is not directly involved in the absorption of vitamin B12.

**Option C:** Somatostatin is a hormone secreted by D cells in the pancreas and gastrointestinal tract that inhibits the secretion of various hormones, including gastrin and secretin. Somatostatin has a negative feedback effect on gastric secretion but is not directly involved in vitamin B12 absorption.

**Option D:** Cholecystokinin (CCK) is a hormone secreted by the duodenum in response to fat and protein ingestion that stimulates the release of pancreatic enzymes and bile from the gallbladder. While CCK plays a role in the digestion of fats and proteins, it is not directly involved in the absorption of vitamin B12.

**Clinical Pearl / High-Yield Fact**
A deficiency in intrinsic factor can lead to vitamin B12 deficiency, which can cause megaloblastic anemia, neuropathy, and demyelination. This highlights the importance of intrinsic factor in maintaining normal vitamin B12 levels.

**✓ Correct Answer: A. Intrinsic factor**