A previously healthy 4-year-old boy presented to an orthopedics clinic with a limp and left knee pain. He reported intermittent pain since falling from his scooter 4 weeks ago and that his symptoms acutely worsened after “tweaking” this knee running 3 days ago. Physical examination revealed a small left knee effusion and limited ability to bear weight, but normal range of motion without erythema, warmth, or fever. Plain radiographs and white blood cell count (5.9 × 109/L) were normal, with an undetectable C-reactive protein (CRP) (<0.4 mg/dL). However, his erythrocyte sedimentation rate (ESR) was mildly elevated to 20 (normal, 0-10 mm/h), prompting referral to the emergency department, where knee ultrasound showed soft-tissue swelling consistent with recent trauma and no obvious effusion. Joint aspiration under sedation was performed and was reassuring against septic arthritis with a leukocyte count of 825/µL. However, he was started on broad-spectrum antibiotics and admitted...

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