Which enzyme initiates protein digestion?

Options

A. (a) Pepsin
B. (b) Trypsin
C. (c) Aminopeptidase
D. (d) Carboxypeptidase

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks which enzyme initiates the process of protein digestion.

2. **Identifying the Enzymes**: The options provided are:
- A) Pepsin
- B) Trypsin
- C) Aminopeptidase
- D) Carboxypeptidase

3. **Function of Each Enzyme**:
- **Pepsin**: This enzyme is known to initiate protein digestion in the stomach.
- **Trypsin**: This enzyme continues protein digestion but is not the initiator; it acts in the small intestine.
- **Aminopeptidase**: This enzyme further breaks down peptides into amino acids but does not initiate protein digestion.
- **Carboxypeptidase**: Similar to aminopeptidase, it acts on peptides but does not start the digestion process.

4. **Role of Pepsin**: Pepsin is secreted by the gastric glands as an inactive form called pepsinogen. This inactive form is activated by the acidic environment of the stomach, primarily due to hydrochloric acid (HCl).

5. **Activation of Pepsin**: Once pepsinogen is activated into pepsin, it begins the process of breaking down proteins into smaller peptides and peptones.

6. **Conclusion**: Based on the information, the enzyme that initiates protein digestion is **Pepsin**.

### Final Statement:
Hence, the correct answer is **Option A: Pepsin**.

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