Final answer:
The attachment and entry stages are critical in the viral replication cycle because attachment initiates the infection and entry allows the viral nucleic acid to begin taking over the host's cellular mechanisms.
Step-by-step explanation:
The step in the replication cycle of viruses that is most critical for the virus to infect cells is the attachment and entry stage. This stage is essential because a virus cannot begin the infection process if it does not first attach to a host cell. This attachment is very specific and involves the virus recognizing and binding to receptor sites on the cell membrane. If the virus fails to attach, it can't penetrate the cell membrane and deliver its nucleic acid into the host, thus making this step pivotal for subsequent stages such as replication of the viral genome, assembly of new virions, and release of mature virions. Without attachment, the entire replication cycle cannot proceed.
During the entry step, the viral nucleic acid needs to enter the host cell, which involves leaving the protective protein capsid outside or being brought into the cell along with the capsid. This step is crucial because it marks the beginning of the virus’s control over the host cell's molecular machinery, leading to the production of new virions.
The final release step is also critical as it allows newly formed virions to infect adjacent cells and continue the replication cycle. This step can happen through cell death or budding, depending on the type of virus, but the failure to release new virions would prevent the infection from spreading.
The correct option is A.