A patient presents with deep venous thrombosis after a long flight. What is the mechanism of action of the anticoagulant that should be used?

• A. Direct thrombin inhibition
• B. Inhibition of factor Xa directly by a small molecule
• C. Activation of antithrombin III
• D. Platelet aggregation inhibition

Answer: (C)

Immobilization-related DVT is typically treated with heparin, whose action centers on antithrombin III. Heparin binds to antithrombin III and markedly increases its ability to inactivate key coagulation enzymes, especially thrombin (factor IIa) and factor Xa. By boosting ATIII activity, the clotting cascade is dampened and fibrin formation is reduced. This mechanism is why the anticoagulant described here is activation of antithrombin III. Other options describe direct inhibition of thrombin or Xa or platelet-targeted effects, which are not how heparin achieves its anticoagulant effect.