This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Systemic Bacteriology (Gram Positive Bacilli, Gram Negative Bacilli), Subject name: Microbiology
Agglutination with 'O' antigen of S. typhi is inhibited by :- 
Select and explain. 
A. Vi Ag, B. Flageller, C. F antigen, D. All the above
A: Vi Ag
Answer: A: Vi Ag 
Explanation: Agglutination with 'O' antigen of S. typhi is inhibited by Vi antigen. Vi antigen: Surface polysaccharide envelope or capsular antigen covering the O antigen. Prevents its agglutination with O antiserum. Phage typing of S. typhi can be done by using Vi specific bacteriophages. Used for vaccination.