Which agent is considered "fibrin selective" and activates plasminogen bound to fibrin?

Alteplase is a thrombolytic agent that specifically targets fibrin, making it fibrin-selective. It acts by converting plasminogen to plasmin, but it does so preferentially when plasminogen is bound to fibrin. This characteristic is significant because it allows alteplase to focus its action on blood clots (thrombi) that are composed primarily of fibrin, enhancing the breakdown of the clot while minimizing systemic activation of the fibrinolytic system. This selectivity helps reduce the risk of bleeding complications that could arise from more non-specific thrombolytics.

The other options provided are anticoagulants or agents that do not have the specific fibrin-selectivity characteristic. Fondaparinux is a synthetic anticoagulant that works primarily by inhibiting factor Xa, while argatroban and bivalirudin are direct thrombin inhibitors that prevent the conversion of fibrinogen to fibrin but do not selectively activate plasminogen bound to fibrin. Therefore, alteplase stands out as the agent specifically designed to act on fibrin-bound plasminogen, making it the correct choice for this question.