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. Step 1: Understand the concept of natural selection. Natural selection is a process where individuals with traits that provide a survival or reproductive advantage are more likely to pass on those traits to the next generation. In the context of HIV, this means that viruses with mutations that confer drug resistance are more likely to survive and replicate in the presence of the drug.
2. Step 2: Recognize that mutations in HIV occur naturally during replication. HIV has a high mutation rate due to the error-prone nature of its reverse transcriptase enzyme. This means that even before treatment begins, there may already be a small population of viruses with mutations that make them resistant to 3TC.
3. Step 3: Analyze the effect of the drug 3TC on the HIV population. When treatment with 3TC begins, the drug inhibits the replication of non-resistant viruses. However, viruses with the resistance mutation are not affected by the drug and continue to replicate.
4. Step 4: Consider the role of selective pressure. The presence of 3TC creates a selective pressure that favors the survival and replication of resistant viruses. Over time, the resistant viruses become the dominant population because they are the only ones able to replicate effectively in the drug's presence.
5. Step 5: Evaluate the options provided in the question. Based on the principles of natural selection and mutation, the best explanation is that a few drug-resistant viruses were present at the start of treatment, and natural selection increased their frequency (option c).

Key Concepts

Natural Selection

Natural selection is a process 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 viral population.

Viral Mutation and Resistance

Viruses, including HIV, can mutate rapidly, leading to variations in their genetic makeup. Some of these mutations may confer resistance to antiviral drugs, allowing those resistant strains to thrive when the drug is administered, as seen with 3TC treatment.

Reinfection with Resistant Strains

Reinfection occurs when a patient contracts a new strain of a virus after being infected with a different strain. In the case of HIV, if a patient becomes reinfected with a strain that is already resistant to 3TC, this could explain the rapid emergence of a resistant viral population.