A 12-year-old boy with sickle cell anemia complains of 24 hours of pain in the right thigh. He denies any traumatic injury. The right leg has tenderness with palpation of the mid-thigh. The patient's temperature is 100.2 degrees F, and ESR is 45. Radiographs of the femur are unremarkable. A radionuclide bone scan demonstrates abnormal uptake in the mid-femur. A radionuclide bone marrow scan demonstrates decreased uptake within the marrow. Which of the following is the best step in management?