This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Communicable diseases, Subject name: Social & Preventive Medicine
Antigenic variation seen in which of the following- 
Select and explain. 
A. Influenza virus, B. Hepatitis virus, C. Yellow fever virus, D. Leptospira
A: Influenza virus
Answer: A: Influenza virus 
Explanation: The influenza A virus is unique among the viruses because it is frequently subject to antigenic variation, both major and minor. When there is a sudden complete or major change, it is called a shift, and when the antigenic change is gradual over a period of time, it is called a drift. Ref:PARK&;S TEXTBOOK OF PREVENTIVE AND SOCIAL MEDICINE 23rd edition,Page:154