Review Question - QID 218714

A 4-year-old girl comes to the ED with 3-day history of leg pain and progressive inability to bear weight following a trip and fall while playing with her dog. Physical exam is notable for erythema around the distal aspect of her left leg. Temperature is 39.0°C (102.2°F). Laboratory studies show a white blood cell count of 16,200/mm3, serum C-reactive protein level of 15.4 mg/L, and erythrocyte sedimentation rate of 52 mm/hr. Her X-rays are shown in Figure A and B. She is started on an empiric cephalosporin. She appears to improve on day 1, but becomes progressively more lethargic on days 2 and 3 without a significant change in her CRP. What is the most likely diagnosis?