You are answering a USMLE-style medical question. Question: Six days after undergoing an aortic valve replacement surgery, a 68-year-old woman has bluish discolorations around the peripheral intravenous catheters and severe right leg pain. She has a history of hypertension and major depressive disorder. She drinks 3 beers daily. Current medications include hydrochlorothiazide, aspirin, prophylactic unfractionated heparin, and fluoxetine. Her temperature is 37.3°C (99.1°F), pulse is 90/min, and blood pressure is 118/92 mm Hg. Examination shows multiple hematomas on the upper extremities around the peripheral intravenous access sites. There is a healing sternal incision. The lungs are clear to auscultation. Cardiac examination shows an aortic click. The right calf is tender, erythematous and swollen. Laboratory studies show: Hemoglobin 13.1 g/dL Leukocyte count 9,900/mm3 Platelet count 48,000/mm3 Prothrombin time 15 seconds Activated partial thromboplastin time 40 seconds Serum Urea nitrogen 19 mg/dL Creatinine 1.1 mg/dL Alkaline phosphatase 33 U/L AST 26 U/L ALT 33 U/L γ-Glutamyl transferase 45 U/L (N=5–50 U/L) Which of the following is the most likely underlying mechanism of this patient's symptoms?" Options: A) Deficiency of vitamin K-dependent clotting factors B) Phagocytosis of autoantibody-opsonized platelets C) Excess circulating thrombin D) Autoantibody-mediated platelet activation Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.