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: Medicine
[{'content': 'Which of the following best represents the impoance of Rheumatoid factor in rheumatoid ahritis? \nSelect and explain. \nA. Rheumatoid factor is associated with bad prognosis, B. Rheumatoid factor correlates with disease activity, C. Rheumatoid factor is very common in childhood Rheumatoid ahritis, D. Absent rheumatoid factor rules out the diagnosis of Rheumatoid ahritis', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Rheumatoid factor (RF) consists of heterogeneous autoantibodies of the IgM class that react against the Fc region of human IgG. It is found in 70% of rheumatoid ahritis patients and is associated with a worse prognosis, since higher levels of rheumatoid factor are associated with more aggressive disease, rheumatoid nodules and extra aicular manifestations. Ref: Elective Hand Surgery: Rheumatological and Degenerative Conditions, Nerve Compression Syndromes By Michel Merle, Page 97 ; Measuring Immunity: Basic biology and Clinical Assessment By Michael T. Lotz, Page 189', 'role': 'assistant'}]