Which enzyme is most active in the stomach?

• A. Lipase
• B. Amylase
• C. Pepsin
• D. Trypsin

Answer: (C)

In the stomach, protein digestion is driven by pepsin, which is activated by the very acidic environment there. Pepsin is secreted as pepsinogen and turned into active pepsin by hydrochloric acid from the stomach lining, giving it an optimal pH around 1.5–2. This makes it the primary enzyme for breaking down proteins in this part of the digestive tract. The other enzymes listed have different roles or operate best in other parts of the digestive system: amylase targets carbohydrates but is inactivated by stomach acid; lipase digests fats but mainly works with pancreatic lipase in the intestine; trypsin is a pancreatic enzyme that functions in the small intestine after activation. So pepsin is the enzyme most active in the stomach.