A previously healthy 3-year-old girl presents to the pediatric rheumatology clinic for evaluation of 5 to 6 weeks of arthralgia of the left wrist and right elbow. Initial atraumatic right elbow pain prompted her pediatrician to refer her to orthopedic surgery. The orthopedic surgeon’s physical examination revealed decreased right elbow range of motion (ROM), and initial laboratory evaluation was notable for an erythrocyte sedimentation rate of 33 mm/hr (reference range, 0–20 mm/hr), a C-reactive protein level of 1.30 mg/dL (13 mg/L) (reference range, 0–1.0 mg/dL [0–10 mg/L]), and a normal complete blood cell (CBC) count: white blood cell (WBC) count, 9,000/µL (9.0 × 109/L) (reference range, 5,000–17,000/µL [5.0–17.0 × 109/L]); hemoglobin level, 12.3 g/dL (123 g/L) (reference range, 11.5–14.0 g/dL [115–140 g/L]); and platelet count, 442 × 103/µL (442 × 109/L) (reference range, 160–370 × 103/µL [160–370 × 109...

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