A child with acute mono arthritis needs urgent evaluation and treatment. A detailed history and examination should be done. In a well child, history of significant trauma should be obtained foremost, and fractures should be looked for in radiographs. However, a history of trauma does not exclude joint infection.
In a sick child, it is essential to rule out Septic arthritis. Joint fluid aspiration can be done and the fluid should be sent for blood cell count, Gram stain, and culture.
Morning stiffness is a characteristic feature of inflammatory arthritis. Common causes of inflammatory mono arthritis are Reactive arthritis, Tubercular arthritis, Transient Synovitis of Hip joint
After ruling out these conditions, other diagnosis such as bleeding disorders and Juvenile Idiopathic arthritis(JIA) should be kept in mind.
When in doubt, an expert pediatric rheumatologist opinion should be sought for.