**Core Concept**
Atrial fibrillation (AF) increases the risk of thromboembolic events, such as stroke. Anticoagulation is recommended for patients with AF and a history of stroke to prevent recurrent events. However, anticoagulation also increases the risk of bleeding.

**Why the Correct Answer is Right**
Apixaban is a direct oral anticoagulant (DOAC) that selectively inhibits Factor Xa, a key enzyme in the coagulation cascade. By inhibiting Factor Xa, apixaban reduces the formation of thrombin and subsequent clot formation. Apixaban has been shown to be effective in reducing the risk of stroke and systemic embolism in patients with AF, with a favorable safety profile, including a lower risk of intracranial hemorrhage compared to vitamin K antagonists. This makes apixaban an ideal choice for patients with AF and a history of stroke who require anticoagulation.

**Why Each Wrong Option is Incorrect**
**Option A:** Vitamin K antagonists, such as warfarin, are effective anticoagulants for AF but have a narrow therapeutic index and require regular monitoring of international normalized ratio (INR). They also have a higher risk of intracranial hemorrhage compared to apixaban.
**Option C:** Clopidogrel is an antiplatelet agent that inhibits the P2Y12 receptor on platelets. While it can be used in conjunction with aspirin for patients with AF, it is not an effective anticoagulant for preventing stroke.
**Option D:** Ticlopidine is another antiplatelet agent that inhibits ADP receptors on platelets. However, it has a higher risk of neutropenia and thrombotic thrombocytopenic purpura compared to clopidogrel and is not a recommended treatment for AF.

**Clinical Pearl / High-Yield Fact**
When choosing an anticoagulant for patients with AF, consider the patient's risk factors for bleeding, renal function, and concomitant medications. DOACs like apixaban offer a more convenient and safer alternative to vitamin K antagonists in many cases.

✓ Correct Answer: B. Apixaban