What enzyme is responsible for breaking down proteins in the stomach?

Pepsin is the enzyme primarily responsible for breaking down proteins in the stomach. It is secreted as an inactive precursor called pepsinogen by the stomach's lining and becomes active in the acidic environment of the stomach, which is essential for its function. Pepsin specifically targets peptide bonds, which are the connections between amino acids in protein molecules, and begins the process of protein digestion.

While trypsin and chymotrypsin are also enzymes that break down proteins, they function primarily in the small intestine rather than the stomach. Trypsin is activated in the small intestine from its precursor and works in a different pH environment compared to pepsin. Protease is a more general term that refers to any enzyme that breaks down proteins, but it does not specify which enzyme is active in the stomach. Hence, pepsin is the correct answer as it is the primary enzyme performing this crucial role during digestion within the stomach.