The correct answer is E. Chronic atrial fibrillation is a risk factor for clot formation and systemic embolization. This man needs ongoing anticoagulation to prevent possible complications, such as cerebrovascular accidents or mesenteric infarction. Warfarin inhibits gamma-carboxylation of vitamin K-dependent clotting factors and is used for chronic anticoagulation. It is taken orally and has a long half-life. The degree of anticoagulation must be followed by measuring the International Normalized Ratio.
Answer A is incorrect. Aspirin works by irreversibly inhibiting cyclooxygenase, thereby preventing the conversion of arachidonic acid to prostaglandins. The four effects of aspirin are antiplatelet (thereby inhibiting thrombus formation), antipyretic, analgesic, and anti-inflammatory. Aspirin is not used to prevent clot formation in the setting of atrial fibrillation, but rather to prevent myocardial infarction. In the setting of atrial fibrillation, the patient needs a drug that prevents activation of the clotting cascade, whereas aspirin acts only on platelet aggregation.
Answer B is incorrect. Protamine sulfate is used for rapid reversal of heparinization in the setting of overzealous anticoagulation.
Answer C is incorrect. Streptokinase is a thrombolytic used to break down existing clots. This could be used in the setting of a myocardial infarction. It would not be used prophylactically to prevent clot formation in atrial fibrillation.
Answer D is incorrect. Heparin is taken parenterally and is used for immediate, not long- term, anticoagulation. This patient was likely given heparin on admission to the hospital and was then converted to warfarin for chronic anticoagulation before his discharge.