In the clotting mechanism, prothrombin is changed into thrombin in the presence of?

PGIMER Sangrur NO - 2023

Applied Physiology

Easy

Appeared in: PGIMER Sangrur NO - 2023

Explanation

The conversion of prothrombin (Factor II) to thrombin is the central event of the common pathway of blood coagulation.
This reaction is catalyzed by an enzyme complex called prothrombinase, also known as prothrombin activator.
Calcium ions (Ca²⁺), designated as Factor IV, are an essential cofactor required for this enzymatic reaction to proceed efficiently.
The prothrombinase complex itself is composed of activated Factor X (Xa) and activated Factor V (Va) assembled on a phospholipid surface, typically from platelets.

Option A: Fibrinogen is the substrate that thrombin acts upon to form fibrin. It is part of the final step of the cascade, occurring after thrombin has already been formed.
Option B: Thromboplastin (Tissue Factor) initiates the extrinsic pathway, an earlier step. Vitamin K is required for the synthesis of prothrombin in the liver, not for its activation in the blood.
Option C: Thromboplastin is an initiator of the extrinsic pathway, which leads to the eventual formation of prothrombinase. It is not the direct enzyme that converts prothrombin.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Coagulation Cascade: Common Pathway as background academic context rather than a clinical decision trigger.
Understanding this step is fundamental for interpreting coagulation studies like Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT), which measure the integrity of different parts of the cascade.
Many anticoagulant medications work by targeting this pathway. For example, heparin enhances the activity of antithrombin, which inhibits thrombin and Factor Xa, while direct oral anticoagulants (DOACs) can directly inhibit thrombin or Factor Xa.
What if? If a patient has severe liver disease, the synthesis of prothrombin (Factor II) would be impaired. This would lead to a prolonged PT/INR and an increased risk of bleeding, even if prothrombinase and calcium are present in normal amounts.

How to Approach the Question

First, identify the core of the question: what is needed to change prothrombin into thrombin?
Recall the three main stages of the coagulation cascade: 1) Formation of prothrombin activator, 2) Conversion of prothrombin to thrombin, 3) Conversion of fibrinogen to fibrin.
Recognize that the question is asking about Stage 2.
Remember that the key enzyme for this stage is called prothrombin activator or prothrombinase.
Recall that calcium ions (Factor IV) are a crucial cofactor for almost all steps of the clotting cascade.
Evaluate the options based on this knowledge. Option D includes both the specific enzyme (prothrombinase) and the necessary cofactor (calcium ion), making it the most complete and correct answer.

Concept Tested & Keywords

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