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: Pathology
[{'content': "In which of the following rheumatoid factor is not present in serum? \nSelect and explain. \nA. Rheumatoid arthritis, B. Felty's syndrome, C. Still's disease, D. Present in all of the above", 'role': 'user'}, {'content': "Answer: C \nExplanation: Still's disease is a variant of rheumatoid arthritis where rheumatoid factor is not seen.", 'role': 'assistant'}]