This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General anatomy, Subject name: Anatomy
Which of the following agents should not be used for oral anticoagulation in high risk patients with atrial fibrillation 
Select and explain. 
A. Wafarin, B. Dabigatran, C. Rivoraxaban, D. Clopidogrel
D: Clopidogrel
Answer: D: Clopidogrel 
Explanation: Oral anticoagulation in high-risk patients with AF includes vitamin K antagonists or the newer anticoagulants such as thrombin inhibi- tors (dabigatran) or factor Xa inhibitors (rivaroxaban, apixaban), but not antiplatelet agents (aspirin and clopidogrel), which have substantially less effect. Ref Harrison 20th edition pg 1456