This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteria, Subject name: Microbiology
True about protein A of staph aureus: 
Select and explain. 
A. Causes opsonization, B. Binds to Fc portion of IgG, C. Stimulate phagocytosis, D. T-cell mitogen
B: Binds to Fc portion of IgG
Answer: B: Binds to Fc portion of IgG 
Explanation: Ans. is 'b' i.e., Binds to Fc part of IgG(Ref: Ananthanarayan, 9th/e, p. 197, 198)* Protein A is a cell wall component of S aureus strains and is a bacterial surface protein called as microbial surface components recognizing adhesive matrix molecules (MSCRAMMs).Bacterial attachment to host cells is mediated by MSCRAMMs, and these are important virulence factors.Protein A of Staph aureus binds to Fc terminal of IgG (IgG 1, 2 and 4 but not IgG 3) and prevents opsono- phagocytosis by PMNs.* Protein A has become an important reagent in immunology and diagnostic laboratory technology; for example, protein A with attached IgG molecules directed against a specific bacterial antigen agglutinates bacteria that have that antigen ("conglutination").