Mechanism of blood clotting is given below :
An injury or a trauma stimulates the platelets in the blood to release certain factors which activate the mechanism of coagulation . An enzyme complex,
…………P……… is formed by a series of linked enzyme reactions
(...........Q.........Process) involving a number of factors present in the plasma in an inactive state. This enzyme complex is required for formation of ...............R............
..........S........... ions play a very important role in clotting .
Identify the correct option that represents P, Q, R , and S .

Options

A. `{:(P,Q,R,S),("thrombokinase","inhibitory","prothrombin","magnesium"):}`
B. `{:(P,Q,R,S),("prothrombin","cascade","thrombin","magnesium"):}`
C. `{:(P,Q,R,S),("thrombin","cascade","thrombokinase","calcium"):}`
D. `{:(P,Q,R,S),("thrombin","cascade","thrombin","calcium"):}`

Answer

To solve the question regarding the mechanism of blood clotting, we need to identify the correct terms that fit into the blanks P, Q, R, and S based on the provided information about the coagulation process.

### Step-by-Step Solution:

1. **Understanding the Mechanism of Blood Clotting**:
- When an injury occurs, platelets in the blood release factors that activate coagulation.
- This process involves a series of linked enzyme reactions.

2. **Identifying P**:
- The enzyme complex formed during the coagulation process is known as **thrombin**.
- Therefore, **P = thrombin**.

3. **Identifying Q**:
- The process of forming thrombin through a series of linked enzyme reactions is referred to as the **cascade process**.
- Hence, **Q = cascade**.

4. **Identifying R**:
- The enzyme thrombin is crucial for the conversion of fibrinogen into **fibrin**, which is essential for blood clot formation.
- Thus, **R = fibrin**.

5. **Identifying S**:
- Calcium ions play a vital role in the clotting process, facilitating various steps in the coagulation cascade.
- Therefore, **S = calcium**.

### Final Answer:
- P = thrombin
- Q = cascade
- R = fibrin
- S = calcium