An 11-year-old girl presents to the ED with a 4-day history of right ankle pain. Her pain is worse with movement of the ankle as well as with weight-bearing. Earlier in the week, she had similar symptoms in her left wrist and elbow. She has no history of trauma, fever, or rash. She has had occasional shortness of breath and chest pain over the past week.

Physical examination reveals a well-appearing girl whose axillary temperature is 101.4°F (38.5°C), heart rate is 122 beats/min, respiratory rate is 16 breaths/min, and blood pressure is 97/60 mm Hg. There is swelling around her right ankle as well as tenderness to light touch and pain on flexion and extension of that joint.

Her ankle radiograph is read as normal. Results of laboratory studies include: WBC count, 18.4×103/mcL (18.4×109/L), with 79% neutrophils, 14% lymphocytes, and 7% monocytes; Hgb, 9.7 g/dL (97...

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