Causes of antigenic drift in influenza viral infections:

Ans. is 'a' i.e., Small mutations in neuraminidase and hemagglutinin Antigenic drift: The gradual, sequential, regular antigenic change in influenza virus is known as antigenic drift. This occurs due to minor antigenic changes due to point mutations in the HA or NA occurring at frequent intervals. This is caused even by a single mutation affecting HA glycoprotein. The antigenic drift is characterized by changes in certain epitopes in the HA, while others are being conserved.