An 8-month-old boy is seen because of a swollen left ankle of several days’ duration. He has had no fever or other signs of illness and has been growing and developing normally.

On physical examination, the infant is alert and cheerful. He has a markedly swollen left ankle and dorsum of the left foot. The ankle is mildly erythematous and warm, with a limited range of motion, and is tender when palpated or moved. The right second toe is edematous, with slight erythema but a normal range of motion. With palpation and passive range of motion, the left ankle is noted to be mildly tender. All other findings are normal.

Laboratory results include: WBC count, 9.1×103/mcL (9.1×109/L) with 52% neutrophils, 40% lymphocytes, 7% monocytes, and 1% eosinophils; Hgb, 12 g/dL (120 g/L); Hct 36% (0.36); platelet count, 454×103/mcL (454×109/L); ESR, 69 mm/h;...

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