Diagnostic challenge

The patient, a 10-year-old female, was referred to the rheumatology clinic with a previous history of fever (39ºC), loss of appetite and polyarthritis of wrist, knees, and ankles. Symptoms were relieved only after using NSAIDs. After 6 months, the patient returned to our hospital with a 7-month history of weight loss and chronic arthritis (claudication and morning stiffness of 15 minutes) on ankles. Physical exam revealed suggestive signs of chronic arthritis with exuberant swelling and tenosynovitis of both ankles. Magnetic resonance imaging of ankles showed bilateral and symmetrical tibiotalar arthritis of extensors, flexors, and fibularis tendons. Right ankle synovial biopsy revealed no granulomas. Which is the most probably diagnostics?