You are answering a USMLE-style medical question. Question: A 75-year-old man presents to the emergency department with a racing heart and lightheadedness for 3 hours. He has had similar episodes in the past, but the symptoms never lasted this long. He denies chest pain, shortness of breath, headaches, and fevers. He had a myocardial infarction 4 years ago and currently takes captopril, metoprolol, and atorvastatin. His pulse is irregular and cardiac auscultation reveals an irregular heart rhythm. Laboratory reports show: Serum glucose 88 mg/dL Sodium 142 mEq/L Potassium 3.9 mEq/L Chloride 101 mEq/L Serum creatinine 0.8 mg/dL Blood urea nitrogen 10 mg/dL Cholesterol, total 170 mg/dL HDL-cholesterol 40 mg/dL LDL-cholesterol 80 mg/dL Triglycerides 170 mg/dL Hematocrit 38% Hemoglobin 13 g/dL Leucocyte count 7,500/mm3 Platelet count 185,000 /mm3 Activated partial thromboplastin time (aPTT) 30 seconds Prothrombin time (PT) 12 seconds Cardiac enzymes Negative An ECG shows the absence of P-waves with an irregular RR complex. A few hours later, his symptoms subside and he is discharged with an additional anticoagulation drug. Which of the following mechanisms explains how this new medication will exert its effects? Options: A) Activation of the antithrombin enzyme B) Inhibition of the cyclooxygenase enzyme C) Inhibition of vitamin K-dependent clotting factors D) Blockage of glycoprotein IIb/IIIa receptors Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.