Background

A healthy 7-year-old Caucasian child presented with swelling and nodular skin lesions around her left knee for 5 days. She reported similar episodes over the last 4 months. There was no history of fever or trauma, and no significant family history.

Examination revealed features of inflammation with significant pain on passive movement, however normal range of joint movement. After admission, she had a temperature >38°C.

An X-ray of the left knee was requested (figure 1). Blood investigations showed a raised serum phosphate level at 3.3 mmol/L, with normal calcium, alkaline phosphatase, parathyroid hormone and vitamin D. Initial C reactive protein was 38 mg/L, rising to 160 mg/L 24 hours later, with an unremarkable full blood count. She was started on intravenous flucloxacillin, and …