Final answer:
M protein is a major virulence factor of S. pyogenes that helps the bacteria to avoid phagocytosis and is a basis for subclassification of group A streptococci based on their virulence and infectivity.
Step-by-step explanation:
The M protein, associated with S. pyogenes (group A Streptococcus), serves as a major virulence factor by helping the bacteria evade the immune system. It does this by altering the surface properties of the bacteria, hindering phagocytosis by blocking the binding of complement molecules. M protein plays a crucial role in the ability of S. pyogenes to cause a range of human infections, including but not limited to strep throat, scarlet fever, and necrotizing fasciitis. Variations in M proteins underlie the classification of different strains within group A Streptococcus.
In answer to the student's question: M protein is 'B. Is a major virulence factor of S. pyogenes.'