A 58-year-old Indian male with atrial fibrillation is on warfarin (INR target 2–3) for stroke prevention. He develops a urinary tract infection and is prescribed ciprofloxacin 500 mg twice daily. One week later, his INR is found to be 5.2 with spontaneous bruising on his arms. The mechanism of this adverse interaction involves the structure marked **B** in the diagram. Which of the following best explains the clinical consequence?