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
True about protein A of staph aureus ? 
Select and explain. 
A. Causes opsonization, B. Binds to Fc pa of IgG, C. Stimulate phagocytosis, D. T-cell mitogen
B: Binds to Fc pa of IgG
Answer: B: Binds to Fc pa of IgG 
Explanation: Ans. is 'b' i.e., Binds to Fc pa of IgG Protein 'A' binds to Fc terminal of IgG (IgG 1, 2 & 4 but not IgG3) and prevents opsonophagocytosis by PMNs.It is a B-cell mitogen. It is chemotactic, anti-complementory and antiphagocytic.