Within a few weeks of treatment with the drug 3TC, a patient's HIV population consists entirely of 3TC-resistant viruses. How can this result best be explained? a. HIV can change its surface proteins and resist vaccines. b. The patient must have become reinfected with a resistant virus. c. A few drug-resistant viruses were present at the start of treatment, and natural selection increased their frequency. d. HIV began making drug-resistant versions of its enzymes in response to the drug.

Verified step by step guidance

1. Understand the nature of HIV and its ability to mutate: HIV is a virus known for its high mutation rate, which allows it to quickly adapt to environmental pressures such as antiviral drugs.
2. Recognize the role of natural selection in viral populations: When a drug like 3TC is used, it creates a selective pressure on the virus population. Viruses that are susceptible to the drug are killed, while any resistant variants survive and reproduce.
3. Identify the initial presence of resistant viruses: Even before treatment, it is likely that a few viruses in the vast population may carry mutations that confer resistance to the drug.
4. Consider the growth of the resistant population: As the treatment kills off the susceptible viruses, the few resistant ones will continue to replicate and eventually come to dominate the population.
5. Evaluate the options given in the problem: Based on the understanding of viral mutation and natural selection, determine which option best explains the scenario where the HIV population becomes entirely resistant to 3TC within a few weeks.

Key Concepts

Natural Selection

Natural selection is a fundamental mechanism of evolution where organisms better adapted to their environment tend to survive and produce more offspring. In the context of HIV treatment, if a few drug-resistant viruses are present before treatment, they can survive the selective pressure of the drug, leading to an increase in their frequency in the population over time.

Viral Mutation and Resistance

Viruses, including HIV, can mutate rapidly due to their high replication rates. These mutations can lead to drug resistance, where some viral strains develop the ability to survive despite the presence of antiviral drugs. This phenomenon is critical in understanding how a patient's HIV population can become entirely resistant to a drug like 3TC within a short period.

Reinfection and Viral Diversity

Reinfection occurs when an individual is infected with a different strain of a virus after an initial infection. In the case of HIV, the presence of multiple strains can lead to increased viral diversity, which may include drug-resistant variants. However, in this scenario, the rapid emergence of resistance is more likely due to pre-existing resistant strains rather than reinfection.