Which strain of avian influenza is most likely to cause highly pathogenic avian influenza?

The choice indicating H5N1 and H7N7 is correct because these specific strains of avian influenza are known for their potential to cause highly pathogenic avian influenza (HPAI). H5N1 has been recognized as one of the most significant viruses affecting domestic and wild birds, often resulting in severe disease and high mortality rates in affected populations. It has a history of spillover into humans, raising public health concerns.

H7N7 is also classified as a highly pathogenic strain associated with severe disease outbreaks in poultry. The ability of both H5 and H7 strains to mutate and increase virulence is a key characteristic that contributes to the severity of the disease they cause.

In contrast, the other strain combinations listed do not have the same associated risk for HPAI. H1N1 and H3N2 are primarily associated with swine and human influenza, and have not been linked to significant outbreaks of highly pathogenic disease in avian populations. Other options, such as H5N3 and H7N2, do not possess the documented virulence and pathogenic characteristics found in H5N1 and H7N7.