A 7-year-old boy from Mumbai presents with a 10-day history of fever (38.5°C), severe pain in the right knee, and refusal to bear weight. His mother reports he had a minor fall 2 weeks ago but did not sustain an obvious injury. On examination, the knee is swollen, warm, and tender; there is no obvious wound. Laboratory investigations show WBC 14,500/μL, CRP 8.2 mg/dL, and ESR 65 mm/h. Blood culture is negative. X-ray of the right knee shows soft tissue swelling but no bone lesion. MRI reveals a 3 cm × 2 cm area of T2 hyperintensity in the metaphyseal region of the distal femur with surrounding marrow edema and periosteal reaction. What is the most likely diagnosis?