A 9-year-old boy presented to the rheumatology consultation with left elbow pain for the prior three months. The pain was associated with swelling, stiffness, and limitation of movement suggestive of inflammation. The blood work was normal. Mono-articular juvenile idiopathic arthritis (JIA) was suspected, and the patient was referred for left elbow ultrasound, which confirmed synovial thickening (arrow on Figure 1A). Treatment with nonsteroidal anti-inflammatory drugs was started, and three weeks later an intra-articular infiltration with cortico-steroids was carried out. Three months later, as the symptoms persisted, a new ultrasound of the elbow was performed, demonstrating joint effusion (asterisk on Figure 1B); elbow radiograph on the same day showed metaphyseal sclerosis (blue asterisk on Figure 2A), peri-articular osteopenia (green asterisk on Figure 2A) and periosteal apposition (arrows
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