This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous, Subject name: Medicine
Rheumatoid factor is 
Select and explain. 
A. IgM Ab against Fc fragment of IgG, B. IgG Ab against Fc fragment of IgG, C. IgM Ab against Fc fragment of IgA, D. IgM Ab against Fc fragment of IgM
A: IgM Ab against Fc fragment of IgG
Answer: A: IgM Ab against Fc fragment of IgG 
Explanation: (A) IgM Ab against Fc fragment of IgG # Rheumatoid factor is a IgM Ab against Fc fregment of IgG.> Serum IgM RF has been found in 75-80% of patients with RA; therefore, a negative result does not exclude the presence of this disease. It is also found in other connective tissue diseases, such as primary Sjogren's syndrome, systemic lupus erythematosus, and type II mixed essential cryoglobulinemia, as well as chronic infections such as subacute bacterial endocarditis and hepatitis B and C. Serum RF may also be detected in 1-5% of the healthy population.Mnemonic: Man Towards Girl: IgM Ab against Fc fragment of IgG.