A patient has been taking warfarin (Coumadin) for recurrent…

A patient has been taking warfarin (Coumadin) for recurrent deep vein thrombosis for more than a year.  She presents today with increased bruising and bleeding gums when she brushes her teeth.  On further questioning she indicates that she has been taking Tagamet HB (cimetidine) an over-the-counter medication for heartburn.  Which of the following is the most likely reason for her symptoms?