This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteriology, Subject name: Microbiology
[{'content': 'Which of the following factor is mainly responsible for virulence in Streptococcus \nSelect and explain. \nA. Carbohydrate, B. Streptokinase, C. Streptodornase, D. M protein', 'role': 'user'}, {'content': "Answer: D \nExplanation: M proteins: The most impoant protein used for typing as well as for virulence. It acts as a virulence factor by inhibiting phagocytosis. It is antigenic. The antibody to the M protein promotes phagocytosis of the coccus and is therefore protective. M protein is heat and acid stable but susceptible to tryptic digestion. It can be extracted by Lancefield's acid extraction method and typing is done with type-specific sera. About 80 M protein types have been recognized. Reference: Textbook of Microbiology; Anathanarayan and paniker's; 10th edition; Page no: 213", 'role': 'assistant'}]