**Core Concept**
Pepsinogen activation is a critical step in the digestion of proteins in the stomach. Pepsinogen is a zymogen, an inactive precursor to the enzyme pepsin, which breaks down proteins into smaller peptides.

**Why the Correct Answer is Right**
Pepsinogen is activated by the enzyme pepsin itself, in a process known as autolysis. When pepsinogen comes into contact with the acidic environment of the stomach, it undergoes a conformational change, allowing pepsin to cleave itself and become active. This self-catalytic process enables pepsinogen to convert into pepsin, which then continues to digest proteins in the stomach.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastric amylase is not involved in the activation of pepsinogen. It is responsible for breaking down carbohydrates, not proteins.
**Option B:** Hydrochloric acid (HCl) creates an acidic environment in the stomach, but it does not directly activate pepsinogen.
**Option C:** Gastric lipase is involved in the breakdown of fats, not the activation of pepsinogen.

**Clinical Pearl / High-Yield Fact**
Remember that pepsinogen activation is a self-catalytic process, meaning that pepsinogen converts into pepsin, which then continues to activate more pepsinogen. This creates a positive feedback loop that amplifies protein digestion in the stomach.

**Correct Answer: C. Gastric lipase is not involved in the activation of pepsinogen.