Which of the following is an example of an anticoagulant?

Heparin is an anticoagulant that works by inhibiting the formation of blood clots, making it crucial in the prevention and treatment of thromboembolic disorders. It functions mainly by enhancing the activity of antithrombin III, a protein that plays a major role in controlling blood coagulation. By doing this, heparin helps to prevent the conversion of fibrinogen to fibrin, thereby reducing clot formation.

In a clinical setting, heparin is often used during procedures that have a high risk of blood clot formation and in patients who may be prone to pulmonary embolism or deep vein thrombosis. Its immediate action and reversibility with protamine sulfate further highlight its significance in acute care situations where rapid anticoagulation is necessary.

Other options mentioned, while important in their own right, serve different therapeutic purposes. For instance, digoxin is primarily used for heart rate control and improving heart contractility, dopamine is used for its hemodynamic effects, particularly in shock and heart failure, and streptokinase is a thrombolytic agent that dissolves existing blood clots rather than preventing their formation.