Advanced Dental Admission (ADAT) Practice Test

Antithrombin is a key component of the body's anticoagulant system, responsible for inhibiting thrombin and factor Xa, thereby regulating blood clotting. The activity of antithrombin is significantly enhanced by heparin.

Heparin is a glycosaminoglycan that accelerates the action of antithrombin by causing a conformational change in the antithrombin molecule. This alteration allows antithrombin to interact more effectively with its target enzymes, resulting in a more potent anticoagulant effect. Specifically, heparin binds to antithrombin and facilitates its reaction with thrombin and factor Xa, leading to increased inhibition of these coagulation factors.

Calcium, although important for various coagulation processes, does not directly enhance antithrombin activity. Vitamin K is involved in the synthesis of clotting factors and does not influence antithrombin's function. Protamine, on the other hand, is primarily used to neutralize heparin but does not enhance antithrombin activity itself. Thus, heparin is the substance that enhances the activity of antithrombin, making it the correct choice.