A 68-year-old woman presents to the emergency department with rapidly worsening shortness of breath over the last 24 hours. She now feels breathless even at rest and has had two episodes of near-syncope. She has a St. Jude mechanical mitral valve, placed 10 years ago for rheumatic heart disease. Her past medical history is significant for persistent atrial fibrillation, for which she is prescribed warfarin. She is not on any other drugs. She recently returned from a month-long trip abroad and admits to missing several doses of warfarin and not having her international normalized ratio (INR) checked for five weeks. Her family history is unremarkable. She only drinks socially and does not smoke.