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: Microbiology
Rheumatoid factors are immunoglobulins directed against: 
Select and explain. 
A. The Fc poion of immunoglobulin G (IgG), B. The Fc poion of immunoglobulin A (IgA), C. The Fc poion of immunoglobulin M (IgM), D. The Fc poion of immunoglobulin D (IgD)
A: The Fc poion of immunoglobulin G (IgG)
Answer: A: The Fc poion of immunoglobulin G (IgG) 
Explanation: The Fc poion of immunoglobulin G (IgG)