Heparin acts via activation of :
**Core Concept**
Heparin is an anticoagulant medication that prevents the formation of blood clots by inhibiting the coagulation cascade. Its mechanism of action involves the activation of a specific enzyme that regulates the coagulation process.
**Why the Correct Answer is Right**
Heparin acts by activating antithrombin III, a naturally occurring inhibitor of the coagulation cascade. When heparin binds to antithrombin III, it forms a complex that inactivates several enzymes involved in blood clot formation, including thrombin and factor Xa. This leads to a decrease in the formation of fibrin clots and an increase in the dissolution of existing clots. The activation of antithrombin III by heparin is a key mechanism by which heparin exerts its anticoagulant effects.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because heparin does not directly activate protein C, which is another enzyme involved in the regulation of blood clotting.
**Option B:** This option is incorrect because heparin does not directly activate plasminogen, which is an enzyme involved in the breakdown of blood clots.
**Option C:** This option is incorrect because heparin does not directly activate factor VIIa, which is an enzyme involved in the initiation of the coagulation cascade.
**Clinical Pearl / High-Yield Fact**
Heparin's mechanism of action is unique in that it requires the presence of antithrombin III to exert its anticoagulant effects. This is in contrast to other anticoagulant medications, such as warfarin, which directly inhibit the production of vitamin K-dependent clotting factors.
**Correct Answer: A. Heparin activates antithrombin III.**