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

Explanation

**Step-by-Step Solution:**

1. **Understanding Protein Hormones**: Protein hormones are large molecules that cannot easily pass through the cell membrane. They exert their effects by binding to specific receptors on the surface of target cells.

2. **Role of Second Messengers**: When a protein hormone binds to its receptor, it triggers a cascade of intracellular events. This often involves the activation of second messengers, which are small molecules that relay signals inside the cell.

3. **Identifying Second Messengers**: Common second messengers include cyclic AMP (cAMP), calcium ions (Ca²⁺), and inositol triphosphate (IP3). These molecules amplify the signal from the hormone and facilitate various cellular responses.

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