**Core Concept**
The chief cells of the stomach secrete pepsinogen, a precursor to pepsin, which is a key enzyme involved in protein digestion. Pepsinogen is secreted into the stomach lumen where it is activated to pepsin by hydrochloric acid (HCl).

**Why the Correct Answer is Right**
Pepsinogen is secreted by the chief cells (also known as peptic cells) of the gastric glands in the stomach. When pepsinogen comes into contact with HCl in the stomach lumen, it is activated to form pepsin, a potent proteolytic enzyme. Pepsin then breaks down dietary proteins into smaller peptides and amino acids, facilitating their absorption in the small intestine.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastrin is a hormone secreted by G cells in the stomach that stimulates the release of gastric acid and pepsinogen, but it is not secreted by chief cells.
**Option B:** Mucous cells in the stomach secrete mucus that protects the gastric mucosa from acid damage, but they do not secrete pepsinogen.
**Option C:** Parietal cells in the stomach secrete HCl and intrinsic factor, but they do not secrete pepsinogen.

**Clinical Pearl / High-Yield Fact**
The chief cells are sensitive to the toxic effects of NSAIDs, which can cause mucosal damage and bleeding in the stomach. This is because NSAIDs inhibit the production of prostaglandins, which protect the gastric mucosa.

**Correct Answer: A. Pepsinogen**