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
Rheumatoid factor is: 
Select and explain. 
A. IgG, B. IgM, C. IgD, D. IgM
D: IgM
Answer: D: IgM 
Explanation: Answer is D (IgM): Rheumatoid factor is. an IgM antibody directed against the Fc fragment of IgG