This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Orthopaedics
[{'content': 'Rheumatoid factor in rheumatoid arthritis is important because - \nSelect and explain. \nA. RA factor is associated with bad prognosis, B. Absent RA fact6r rules out the diagnosis of Rheumatoid arthritis, C. It is very common in childhood Rheumatoid arthritis, D. It correlates with disease activity', 'role': 'user'}, {'content': 'Answer: A \nExplanation: "The chief value of the rheumatoid factor tests is in the assessment of prognosis : persistently high titers herold more\xa0serious disease including extra-articular features".\xa0— Apley', 'role': 'assistant'}]