A 10 year old has had a painful non-erythematous swelling in the right knee for 4 days. The pain is worse in the morning and with exercise. The family denies fever or flulike symptoms. The family history is unremarkable for arthritis in the maternal side and it was noted that they went for a hiking trip in Massachusetts 2 months ago. The rest of the history and PE is unremarkable.
1. What differential diagnosis would be included in the work-up?
2. What diagnosis is most likely?
3. What laboratory test would help rule in or rule out your considered diagnosis?