A 23-year-old woman has 2-month history of increasing malaise, decreased exercise tolerance, and darken urine. She has new onset joint pain and skin rash on her face. On examination she is jaundiced, she has splenomegaly on abdominal examination. Hemoglobin: 6.9 g\dl Leukocytosis: 6500 Platelet: 250.000\ul Coombs test: positive for igG Which of the following is the single most likely cause of her presentation?