Final answer:
AZT-resistant virions increase in the presence of AZT because AZT provides selection pressure that favors these resistant strains. ART, which uses a combination of drugs, makes it harder for the virus to develop simultaneous resistance, but resistant strains can still emerge.
Step-by-step explanation:
Antiviral drug resistance is a significant challenge in the treatment of HIV, a retrovirus that can mutate quickly, leading to the development of drug-resistant strains. AZT (azidothymidine), a reverse transcriptase inhibitor, was an early drug used in the fight against HIV/AIDS. The presence of AZT imposes selection pressure on HIV, favoring virions (virus particles) that have developed resistance to the drug through mutations. As these resistant virions can replicate even in the presence of the drug, their numbers will increase. Conversely, the removal of AZT from the treatment regime would reduce the selective pressure, potentially leading to a decrease in the proportion of resistant virions.
To combat this issue, ART (Anti-Retroviral Therapy) often involves a combination of drugs, including reverse transcriptase inhibitors, protease inhibitors, integrase inhibitors, and fusion inhibitors. By targeting different stages of the viral life cycle, it becomes more challenging for the virus to develop resistance simultaneously to multiple drugs. However, if resistance does occur to components of the ART regimen, the resistant virions will increase in presence while the therapy is being administered because the drugs provide selective pressure that supports their survival and replication.