Which anticoagulant inactivates factor X and opposes the conversion of prothrombin to thrombin?

1. Both named steps, factor X and the prothrombin to thrombin conversion, sit in the common pathway. 2. Antithrombin is the circulating anticoagulant that acts there, inactivating factor X and opposing that conversion. 3. The protein C system inactivates factors of the intrinsic pathway instead. 4. Tissue factor pathway inhibitor acts on the extrinsic pathway. 5. So each anticoagulant maps to a different limb of the cascade, and the question is answered by locating the steps first. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 18 "The Cardiovascular System: Blood", section 18.5 Hemostasis_