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
[{'content': "Agglutination with 'O' antigen of S. typhi is inhibited by :- \nSelect and explain. \nA. Vi Ag, B. Flageller, C. F antigen, D. All the above", 'role': 'user'}, {'content': "Answer: A \nExplanation: 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.", 'role': 'assistant'}]