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
Clinical significance of Vi antigen of S. typhi is: 
Select and explain. 
A. Helps in diagnosis, B. Highly immunogenic, C. Most impoant antigen for widal test, D. Antibody against Vi-antigen is used for diagnosis of carrier
D: Antibody against Vi-antigen is used for diagnosis of carrier
Answer: D: Antibody against Vi-antigen is used for diagnosis of carrier 
Explanation: Ans. is. 'd' i. e., Antibody against Vi-antigen is used for diagnosis of carrier