Step 1: Understanding prothrombinase formation.
Prothrombinase is a complex enzyme involved in the blood clotting (coagulation) process. It catalyzes the conversion of prothrombin into thrombin. Step 2: Role of thromboplastin.
Thromboplastin (also called tissue factor) is released from damaged tissues or platelets and initiates the cascade of reactions that lead to the formation of prothrombinase. Step 3: Analysis of options. (A) Prothrombin: It is a substrate, not the initiator. (B) Thromboplastin: Correct — it initiates the formation of prothrombinase. (C) Fibrinogen: It is converted into fibrin later in clotting. (D) Heparin: It is an anticoagulant that prevents clotting. Step 4: Conclusion.
The formation of prothrombinase is initiated by thromboplastin.