A 7-year-old boy from Delhi presents with a 10-day history of fever (38.5–39.5°C), severe pain in the right knee, and refusal to bear weight. His mother reports he had a minor fall 2 weeks ago but no open wound. On examination, the knee is swollen, warm, and tender; there is marked limitation of all movements. Laboratory tests show WBC 16,200/μL, ESR 82 mm/hr, CRP 18 mg/dL. Blood culture grows Staphylococcus aureus. Plain radiographs of the knee show soft-tissue swelling and early metaphyseal lucency. MRI reveals a metaphyseal lesion with marrow edema and periosteal reaction in the distal femur, without joint effusion. What is the most likely diagnosis?