Which of the following is absent in the blood?

Options

A. Prothrombin
B. Fibrin
C. Blood platelets
D. Calcium

Answer

To answer the question "Which of the following is absent in the blood?", we will analyze each of the provided options one by one.

### Step-by-Step Solution:

1. **Identify the Options**: The question asks us to determine which of the given options is absent in the blood. The options include prothrombin, fibrin, blood platelets, and calcium.

2. **Evaluate Option 1: Prothrombin**:
- Prothrombin is a plasma protein that is essential for blood clotting. It is synthesized in the liver and is present in the blood.
- **Conclusion**: Prothrombin is **not absent** in the blood.

3. **Evaluate Option 2: Fibrin**:
- Fibrin is an insoluble protein that forms the meshwork of a blood clot. It is produced during the clotting process in response to bleeding.
- Under normal conditions, fibrin is not present in the blood; it is formed only when needed for clotting.
- **Conclusion**: Fibrin is **absent** in the blood under normal conditions.

4. **Evaluate Option 3: Blood Platelets**:
- Blood platelets (thrombocytes) are cell fragments that play a crucial role in blood clotting. They are present in the blood and help prevent bleeding.
- **Conclusion**: Blood platelets are **not absent** in the blood.

5. **Evaluate Option 4: Calcium**:
- Calcium is an essential mineral that plays various roles in the body, including blood clotting, nerve function, and muscle contraction. It is present in the blood.
- **Conclusion**: Calcium is **not absent** in the blood.

### Final Answer:
Based on the evaluations, the correct answer is **fibrin**, as it is the only option that is absent in the blood under normal conditions.

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