**Core Concept**
Zidovudine, a nucleoside reverse transcriptase inhibitor (NRTI), is a key component in the treatment of HIV/AIDS. Resistance to zidovudine develops due to mutations in the HIV-1 reverse transcriptase gene, which alter the enzyme's substrate specificity and reduce the drug's efficacy.

**Why the Correct Answer is Right**
The primary mechanism of resistance to zidovudine involves the substitution of thymidine at position 215 with a different amino acid, such as tyrosine or phenylalanine. This mutation, known as the M41L mutation, leads to a decrease in the affinity of the reverse transcriptase enzyme for zidovudine, rendering the drug less effective. Additionally, other mutations, such as M184V, can also contribute to resistance by altering the enzyme's substrate specificity and reducing the incorporation of zidovudine into the viral DNA.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a particular mutation or mechanism of resistance to zidovudine.
**Option B:** This option is incorrect because it suggests that resistance to zidovudine is due to a mutation in a different gene, rather than the HIV-1 reverse transcriptase gene.
**Option C:** This option is incorrect because it implies that resistance to zidovudine is due to a change in the viral load or replication rate, rather than a specific mutation in the reverse transcriptase enzyme.

**Clinical Pearl / High-Yield Fact**
Monitoring for resistance to zidovudine and other antiretroviral drugs is crucial in the management of HIV/AIDS. Regular genotypic and phenotypic resistance testing can help guide treatment decisions and prevent the development of multidrug-resistant HIV strains.

**Correct Answer:** D.