Review Question - QID 212370

A 66-year-old woman presents to the emergency department complaining of palpitations. She says that she has been experiencing palpitations and lightheadedness for the past 6 months, but before this morning the episodes usually resolved on their own. The patient’s medical history is significant for a transient ischemia attack 2 months ago, hypertension, and diabetes. She takes aspirin, metformin, and lisinopril. She states her grandfather died of a stroke, and her mom has a "blood disorder." Her temperature is 98.7°F (37°C), blood pressure is 138/91 mmHg, pulse is 128/min, respirations are 14/min, and oxygen saturation is 98% on room air. An electrocardiogram is obtained that shows an irregularly irregular rhythm with rapid ventricular response, consistent with atrial fibrillation. She is given intravenous metoprolol, which resolves her symptoms. In addition to starting a beta-blocker for long-term management, the patient meets the criteria for anticoagulation. Both unfractionated heparin and warfarin are started. 5 days later, the patient began complaining of pain and swelling in her left lower extremity. A Doppler ultrasound reveals thrombosis in her right popliteal and tibial veins. A complete blood count is obtained that shows a decrease in platelet count from 245,000/mm^3 to 90,000/mm^3. Coagulation studies are shown below:

Prothrombin time (PT): 15 seconds
Partial thromboplastin time (PTT): 37 seconds
Bleeding time: 14 minutes

Which of the following is the most likely diagnosis?