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: Surgery
A 24-year-old woman presents with a 3-day history of fever and cough with rust-colored sputum. He had splenectomy 1 year ago. CXR film indicates consolidation of right lower lobe. Blood cultures are positive for alpha hemolytic gram-positive diplococci. Immunity to the causative organism is based on 
Select and explain. 
A. Alternative complement pathway activation, B. Antibody to fimbriae, C. IgA antibodies to C carbohydrate, D. IgG antibodies to a surface acidic polysaccharide
D: IgG antibodies to a surface acidic polysaccharide
Answer: D: IgG antibodies to a surface acidic polysaccharide 
Explanation: This is a history of pneumonia due to S. pneumoniae. The most impoant virulence factor of this organism is capsule. Therefore IgG antibodies to the capsular antigen are protective. Other virulence factors produced by pneumococcus include pneumolysin and amidase (also known as suicidal enzyme).