A 24-year-old man presents with fever, malaise, and a petechial rash on his legs and trunk. He reports a recent upper respiratory infection. Examination reveals blood pressure 100/60 mm Hg and scattered petechiae. Laboratory tests show a platelet count of 8,000/microL, normal hemoglobin, normal white cell count, and normal coagulation studies. There is no splenomegaly. HIV and hepatitis C tests are negative. According to immune thrombocytopenia guidelines, what is the most appropriate initial therapy?