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: Surgery
[{'content': 'Rheumatoid factor is: \nSelect and explain. \nA. IgG, B. IgM, C. IgD, D. IgM', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Answer is D (IgM): Rheumatoid factor is. an IgM antibody directed against the Fc fragment of IgG', 'role': 'assistant'}]