Which prevents conversion of prothrombin to thrombin in an undamaged blood vessel ?

Options

A. Heparin
B. Thromboplastin
C. Calcium
D. Fibrinogen

Answer

To answer the question "Which prevents conversion of prothrombin to thrombin in an undamaged blood vessel?", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Process of Coagulation**:
- Coagulation is the process by which blood changes from a liquid to a gel, forming a blood clot. This process involves several steps, including the conversion of prothrombin into thrombin.

2. **Role of Prothrombin and Thrombin**:
- Prothrombin is a plasma protein that is converted into thrombin, an enzyme that plays a crucial role in blood clotting. Thrombin then converts fibrinogen into fibrin, leading to the formation of a clot.

3. **Factors Involved in the Conversion**:
- The conversion of prothrombin to thrombin is facilitated by clotting factors, particularly factor X (also known as prothrombinase).

4. **Identifying Anticoagulants**:
- Anticoagulants are substances that prevent blood clotting. Heparin is a well-known anticoagulant that inhibits the conversion of prothrombin to thrombin.

5. **Evaluating the Options**:
- The options provided are:
- A) Heparin
- B) Thromboplastin
- C) Calcium
- D) Fibrinogen
- Among these, heparin is the only substance that acts as an anticoagulant and prevents the conversion of prothrombin to thrombin.

6. **Conclusion**:
- Therefore, the correct answer to the question is **Heparin (Option A)**, as it prevents the conversion of prothrombin to thrombin in an undamaged blood vessel.