A 5-year-old boy presents with a 2-day history of fever, irritability, and refusal to bear weight on his right leg. On examination, his temperature is 39.0°C, and there is warmth, swelling, and marked tenderness over the distal femur metaphysis. His CRP is 150 mg/L and WCC is 18 x 10^9/L. What is the most likely diagnosis?