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: Microbiology
[{'content': 'Clinical significance of Vi antigen of S. typhi is: \nSelect and explain. \nA. Helps in diagnosis, B. Highly immunogenic, C. Most impoant antigen for widal test, D. Antibody against Vi-antigen is used for diagnosis of carrier', 'role': 'user'}, {'content': "Answer: D \nExplanation: Ans. is. 'd' i. e., Antibody against Vi-antigen is used for diagnosis of carrier", 'role': 'assistant'}]