You are answering a USMLE-style medical question. Question: A 29-year-old female presents to her primary care provider with an aching pain in her left leg that has progressively gotten worse over several days. She recently had an unfortunate sporting incident resulting in large bruises along both legs and lower thighs. An X-ray after the event was negative for fracture. Past medical history is positive for systemic lupus erythematosus. She also has a history of one spontaneous abortion at 12 weeks gestation. Today, her heart rate is 90/min, respiratory rate is 17/min, blood pressure is 119/80 mm Hg, and temperature is 37.0°C (98.6°F). On physical examination, her left leg appears pink and slightly swollen. Homan’s sign is positive. A Doppler ultrasound reveals a thrombus in the left popliteal vein. Her lab results are as follows: Platelets 250,000/mm3 Prothrombin Time 14 sec Partial Thromboplastin Time 90 sec Mixing study (PTT) 89 sec What is the most likely cause of the patient’s condition? Options: A) Antibodies directed against platelet glycoprotein IIb/IIIa B) Antibodies directed against phospholipids C) Antibodies directed against endothelial cells D) Vitamin K deficiency Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.