This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Microbiology
[{'content': 'Cause of antigenic drift in influenza viral infections is \nSelect and explain. \nA. Small mutation in neuraminidase and hemagglutinin, B. Large mutations in hemagglutinin only, C. Step mutations in viral genome, D. None of the above', 'role': 'user'}, {'content': 'Answer: A \nExplanation: antigenic drift - small changes in the genes of influenza viruses that happen continually over time as the virus replicates.Antigenic shift - an abrupt, major change in the influenza A viruses, resulting in new hemagglutinin and neuraminidase proteins in influenza viruses that infect humans.', 'role': 'assistant'}]