Fondaparinux exerts its anticoagulant effect by enhancing antithrombin-mediated inhibition of which coagulation factor?

• A. Directly inhibits thrombin
• B. Binds to antithrombin to enhance inhibition of factor Xa
• C. Inhibits vitamin K epoxide reductase
• D. Activates plasminogen to plasmin

Answer: (A)

Fondaparinux works by binding to antithrombin and greatly increasing its ability to inhibit factor Xa. This synthetic pentasaccharide is structurally suited to accelerate AT’s inhibition of Xa specifically, which lowers thrombin generation downstream and slows clot formation. It does not directly inhibit thrombin, and its short chain length prevents it from bridging AT to thrombin the way larger heparins can. It also has no effect on vitamin K–dependent processes or on plasminogen activation, which are the actions of warfarin and fibrinolytics, respectively.