Resistance to zidovirdine develops due to:
**Core Concept**
Zidovudine (AZT) is a nucleoside reverse transcriptase inhibitor (NRTI) used in the treatment of HIV/AIDS. Resistance to zidovudine develops through mutations in the HIV reverse transcriptase gene, leading to reduced drug efficacy.
**Why the Correct Answer is Right**
The primary mechanism of zidovudine resistance is through the development of mutations in the HIV-1 reverse transcriptase gene, particularly at positions 41, 64, 75, 97, 100, 101, 103, 106, 108, 181, 184, 188, 190, 215, and 219. These mutations alter the binding affinity of zidovudine to the reverse transcriptase enzyme, reducing its effectiveness. The most common mutation is a substitution at position 184, where thymidine is replaced by a methionine (184M) or valine (184V) residue. This mutation reduces the binding affinity of zidovudine to the enzyme, leading to reduced drug efficacy.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because zidovudine resistance is not primarily due to alterations in the host cell's nucleoside salvage pathway. While the host cell's nucleoside salvage pathway plays a role in the activation and metabolism of zidovudine, mutations in this pathway are not the primary cause of resistance.
* **Option B:** This option is incorrect because zidovudine resistance is not due to the development of a zidovudine-resistant strain of the host cell. The host cell is not directly involved in the development of resistance to zidovudine.
* **Option C:** This option is incorrect because zidovudine resistance is not due to the development of a zidovudine-resistant strain of the virus. While the virus does develop resistance to zidovudine through mutations, this is not the same as the development of a resistant strain.
**Clinical Pearl / High-Yield Fact**
It is essential to monitor for zidovudine resistance in patients undergoing treatment for HIV/AIDS, particularly those with a history of prolonged treatment or those who have received multiple antiretroviral therapies. Regular viral load and resistance testing can help identify patients who may require alternative therapies.
**Correct Answer: D. Mutations in the HIV-1 reverse transcriptase gene.**