**Core Concept**
Direct fibrinolytics are a class of drugs that directly dissolve fibrin clots by activating plasminogen to plasmin. They are used to treat acute ischemic strokes and pulmonary embolism.

**Why the Correct Answer is Right**
Alteplase (also known as Activase or Alfimiprase) is a direct fibrinolytic that works by binding to fibrin in a clot and activating plasminogen to plasmin, directly dissolving the clot. This mechanism is distinct from indirect fibrinolytics, which activate plasminogen to plasmin indirectly. Alteplase is administered intravenously and is used to treat acute ischemic strokes within 3-4.5 hours of symptom onset. Its direct action on fibrin makes it an effective treatment for clot dissolution.

**Why Each Wrong Option is Incorrect**
**Option A:** Reteplase is an indirect fibrinolytic, which activates plasminogen to plasmin through a non-fibrin-specific mechanism. It does not directly bind to fibrin.
**Option C:** rtPA (recombinant tissue plasminogen activator) is an indirect fibrinolytic, similar to alteplase but with a different mechanism of action. While it does activate plasminogen to plasmin, it does so through a non-fibrin-specific mechanism.

**Clinical Pearl / High-Yield Fact**
Direct fibrinolytics like alteplase are most effective when administered early in the course of acute ischemic stroke, as delayed treatment can lead to permanent neurological damage.

✓ Correct Answer: B. Alfimiprase