<prompt> Which of the following agents should not be used for oral anticoagulation in high risk patients with atrial fibrillationWafarinDabigatranRivoraxabanClopidogrel  <response>: 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 