In the mechanism of action of a protein hormone, one of the second messengers is

Options

A. Cyclic AMP
B. Insulin
C. `T_3`
D. Gastrin

Answer

To solve the question regarding the second messenger in the mechanism of action of a protein hormone, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Protein Hormones**:
- Protein hormones are large molecules that include amino acid derivatives, peptides, and proteins. Due to their size, they are not soluble in lipids.

**Hint**: Remember that protein hormones cannot easily cross cell membranes due to their size and hydrophilicity.

2. **Mechanism of Action**:
- Since protein hormones cannot enter target cells, they bind to specific receptor molecules located on the cell surface.

**Hint**: Focus on the role of receptors in the action of protein hormones.

3. **Role of Receptors**:
- The binding of the hormone to its receptor triggers a cascade of intracellular events, leading to the generation of second messengers.

**Hint**: Consider what happens after the hormone binds to the receptor.

4. **Identify Second Messengers**:
- Common second messengers generated include cyclic AMP (cAMP), inositol trisphosphate (IP3), and calcium ions (Ca²⁺).

**Hint**: Recall the common second messengers associated with protein hormone signaling.

5. **Evaluate the Options**:
- Given the options:
- A) Cyclic AMP
- B) Insulin
- C) T3
- D) Gastrin
- Insulin, T3, and gastrin are hormones themselves, not second messengers. Cyclic AMP is a well-known second messenger in the signaling pathway of protein hormones.

**Hint**: Distinguish between hormones and second messengers in the options provided.

6. **Conclusion**:
- Therefore, the correct answer to the question is **A) Cyclic AMP**, as it is one of the second messengers involved in the mechanism of action of protein hormones.

### Final Answer:
**A) Cyclic AMP**