Which enzyme begins the breakdown of protein?

Pepsin is the correct answer because it is a digestive enzyme specifically responsible for breaking down proteins into shorter peptides in the stomach. It is produced in an inactive form called pepsinogen, which is activated by the acidic environment of the stomach. Once activated, pepsin effectively cleaves peptide bonds, leading to protein digestion.

The other enzymes mentioned have different functions. Amylase is involved in the breakdown of carbohydrates into simpler sugars, primarily in the mouth and pancreas. Lactase is an enzyme that specifically breaks down lactose, the sugar found in milk, into glucose and galactose. Protease, while it does refer to a broad category of enzymes that break down proteins, is not specific and does not identify the enzyme that acts in the stomach. Pepsin is a specific type of protease, making it the most accurate choice for this question.