Review Question - QID 7236

A 10-year-old girl has had nontraumatic swelling of the left knee for the past month. No other joints are swollen and there is no history of fever, although the patient's mother does recall the child having a localized, but expanding "target-like" rash a few months ago when the family was vacationing in Connecticut. Examination of the knee reveals moderate swelling with no tenderness and near full range of motion. The child lacks perhaps the final 15 degrees of extension and the final 20 degrees of flexion. Laboratory studies show a normal CBC count but the erythrocyte sedimentation rate is 35 mm/h (0-20 normal). Antinuclear antibody test and rheumatoid factor tests are negative. What is the most likely diagnosis?