system
Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
user
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." 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 criteria for anticoagulation. Both unfractionated heparin and warfarin are started. Five days later, the patient begins complaining of pain and swelling of 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?
assistant
Type II heparin-induced thrombocytopenia