An 84-year-old woman with history of atrial fibrillation presents to the emergency department with right arm weakness that began acutely 2 hours earlier, accompanied by dizziness and nausea. She takes warfarin, digoxin, and diltiazem. Vital signs are temperature 37 degrees Celsius, blood pressure 180/100 mm Hg, pulse 120-150 and irregular, respiratory rate 16 per minute, SpO2 98% on 4L oxygen by nasal cannula. Head CT shows no signs of bleed. What is the most likely etiology of her stroke