The substance that keeps prothrombin in inactive state is

Options

A. prothrombin
B. antithrombin
C. thrombokinase
D. thromboplastin

Answer

To solve the question regarding the substance that keeps prothrombin in an inactive state, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Prothrombin**:
- Prothrombin is a protein that plays a crucial role in the blood clotting process. It is converted into thrombin, which is essential for the formation of a blood clot.

2. **Prothrombin Activation**:
- For prothrombin to function in clotting, it must be converted from its inactive form to an active form (thrombin). This conversion is a vital step in the coagulation cascade.

3. **Role of Anti-thrombin**:
- The question asks which substance keeps prothrombin in its inactive state. The substance responsible for this is **anti-thrombin**.
- Anti-thrombin is a protein that inhibits the activity of thrombin and other clotting factors, thereby preventing the premature activation of prothrombin.

4. **Inhibition Mechanism**:
- Anti-thrombin inactivates factor X, which is necessary for the conversion of prothrombin to thrombin. By inactivating factor X, anti-thrombin effectively keeps prothrombin in its inactive state.

5. **Evaluating Other Options**:
- The other options provided (prothrombin, thrombokinase, and thromboplastin) do not keep prothrombin inactive:
- **Prothrombin** is the protein in question.
- **Thrombokinase** and **thromboplastin** are enzymes that facilitate the conversion of prothrombin to thrombin, thus promoting clot formation rather than inhibiting it.

6. **Conclusion**:
- Therefore, the correct answer to the question is **anti-thrombin**.

### Final Answer:
The substance that keeps prothrombin in an inactive state is **anti-thrombin**.