The patient is a 65 year-old woman with a past history of mitral valve regurgitation who had a mitral valve repair with Gore-Tex neochordae and mitral valve annuplasty 2 months earlier. She presented to the emergency department with recent-onset dark (Coca-Cola-colored) urine and fatigue. She was found to be newly anemic with an elevated serum LDH and low haptoglobin, and was admitted for further investigation. She denies a past history of hemolysis, anemia or other hematological condition. She has a history of peptic ulcer disease, hypertension and hyperlipidemia. There is no family history of hematological disorders. She is a non-smoker and drinks the occasional glass of wine. She lives with her husband at home, and is a retired librarian. She is taking atorvastatin, lisinopril, metoprolol and warfarin (for the valve). She has no known drug allergies.
Based on the history (and assuming that the patient does indeed have hemolysis), which of the following diagnosis is most likely?