A 35-year-old woman with SLE presents with severe thrombocytopenia (platelet count 15,000/μL), hemolytic anemia (Hb 7.2 g/dL, reticulocyte count 8%, LDH 680 U/L, negative direct Coombs test), and no clinical evidence of active lupus nephritis or vasculitis. She is seronegative for antiphospholipid antibodies. What is the drug of choice for initial treatment of SLE-associated thrombocytopenia and hemolytic anemia?