A 7-year-old boy developed a limp with right leg pain five days ago, and today has trouble bearing weight. On exam, he is lethargic and has chills. His temperature is 38.4 degrees centigrade. He points to his right inguinal region as the source of the discomfort. He winces with compression of his pelvis. Lab studies reveal a white blood cell count of 11,400/ul, CRP of 0.9 mg/dL (normal < 1.0 mg/dL), and erythrocyte sedimentation rate of 55 mm/h. A pelvis radiograph is shown in Figure A. Ultrasound guided aspiration of the right hip joint yields 9,000 leukocytes per mL. What is the most appropriate next step in management?