Pepsin is an enzyme that digests proteins at acidic pH. Therefore, it must act in theMultiple Choicestomach.small intestine.mouth.large intestine.esophagus.

Break Down the Problem

1. Identify the function of pepsin.
2. Determine the optimal pH for pepsin activity.
3. Locate the human digestive system regions where the pH is acidic.

Relevant Concepts

1. Pepsin is an enzyme specifically responsible for protein digestion.
2. It operates optimally at a very acidic pH, typically around pH 1.5 to 2.5.
3. The primary site of protein digestion in the human body is the stomach, where such acidic conditions are maintained.

Analysis and Detail

1. Among the options provided—stomach, small intestine, mouth, large intestine, and esophagus—the stomach is the only region with an acidic environment due to gastric acid (hydrochloric acid).
2. The small intestine has a more neutral pH due to the bicarbonate ions released from the pancreas.

Verify and Summarize

Pepsin functions effectively in the stomach due to its acidic conditions. In the other regions mentioned, either the pH is too high or does not support pepsin's enzymatic activity.

Final Answer

Pepsin must act in the stomach.