**Question:** Which one of the following preferentially activates plasminogen bound to fibrin and avoids the systemic lytic state:

A. Streptokinase
B. Alteplase
C. Urokinase
D. Tissue Plasminogen Activator (tPA)

**Core Concept:** Plasminogen is a plasma protein that can be converted into the fibrinolytic enzyme plasmin by various activators. In a pathological condition, excessive activation of plasminogen can lead to a systemic lytic state, characterized by uncontrolled fibrinolysis and potential organ damage. Fibrin-bound plasminogen activation is a more specific and targeted approach to fibrinolysis.

**Why the Correct Answer is Right:** B. Alteplase (Actilyse) is a recombinant form of tissue plasminogen activator (tPA). Unlike other options, tPA has a higher affinity for binding to fibrin, the protein mesh formed by the breakdown of blood clots. This makes it preferentially activate plasminogen bound to fibrin while sparing the unbound plasminogen, thus avoiding the systemic lytic state.

**Why Each Wrong Option is Incorrect:**

A. Streptokinase (SK) is a bacterial enzyme that activates both unbound and bound plasminogen, leading to systemic fibrinolysis and potential organ damage.

B. Urokinase (Urokinase) is another activator of plasminogen, but it has a lower affinity for fibrin compared to tPA. This results in less specific activation of plasminogen bound to fibrin and more systemic activation, not suitable for preventing the systemic lytic state.

C. Tissue Plasminogen Activator (tPA) activates both unbound and bound plasminogen, causing systemic fibrinolysis and the risk of organ damage.

**Clinical Pearl:** In clinical settings, tPA is used to dissolve blood clots (thrombolysis) in specific conditions like acute ischemic stroke, acute myocardial infarction, and pulmonary embolism. However, its fibrin-specific activation (as seen with Alteplase) is crucial to prevent the systemic lytic state and associated complications.