An 18-year-old boy presents with pain and swelling of his left thigh that developed suddenly 2 days earlier. He denies any recent trauma to the site, fevers, or weight loss.On examination, an area of swelling is visible over the lower half of his left thigh. No erythema or other associated skin changes are noted. Distal pulses are palpable. The patient has full range of motion.Plain radiographs of the knee are shown.
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