What is the mechanism of action (MOA) of heparin?

The mechanism of action of heparin is primarily based on its ability to enhance the activity of antithrombin III, a natural anticoagulant. When heparin binds to antithrombin III, it induces a conformational change that significantly accelerates its ability to inactivate thrombin (factor IIa) and factor Xa. This inhibition of thrombin is crucial because thrombin is a key enzyme in the coagulation cascade that not only converts fibrinogen into fibrin, leading to clot formation, but also activates other coagulation factors.

By potentiating the action of antithrombin III, heparin effectively reduces the formation of clots, preventing thrombosis and promoting anticoagulation in various clinical settings, such as during surgeries or in patients with certain thromboembolic conditions. This key interaction is why option B accurately describes the predominant mechanism through which heparin exerts its pharmacological effects.

In other contexts, inhibiting platelet aggregation, promoting conversion of fibrinogen to fibrin, or decreasing blood viscosity may relate to different anticoagulant strategies or mechanisms but do not encompass the specific action of heparin as it operates through the antithrombin III pathway.