**Core Concept**
The question is testing the interaction between different anti-HIV drugs, specifically the concept of mutual antagonism. Mutual antagonism occurs when two drugs have opposite effects on the same biological process, leading to a decrease in the efficacy of one or both drugs when used together. In this context, it refers to the interaction between zidovudine (AZT) and another anti-HIV drug.

**Why the Correct Answer is Right**
Zidovudine is a nucleoside reverse transcriptase inhibitor (NRTI) that works by inhibiting the reverse transcriptase enzyme, which is essential for the replication of retroviruses like HIV. The correct answer is likely to be stavudine (d4T), another NRTI. When used together, zidovudine and stavudine can mutually antagonize each other by competing for the same binding site on the reverse transcriptase enzyme, reducing their individual efficacy.

**Why Each Wrong Option is Incorrect**
* **Option A:** Didanosine (ddI) - While didanosine is also an NRTI, it does not exhibit mutual antagonism with zidovudine.
* **Option B:** Lamivudine (3TC) - Lamivudine is a nucleoside reverse transcriptase inhibitor that can be used in combination with zidovudine without mutual antagonism.
* **Option C:** Indinavir (IDV) - Indinavir is a protease inhibitor, a different class of anti-HIV drugs that targets a different step in the HIV replication cycle, and does not exhibit mutual antagonism with zidovudine.

**Clinical Pearl / High-Yield Fact**
When combining anti-HIV drugs, it's essential to consider potential interactions, including mutual antagonism, to maximize efficacy and minimize the risk of resistance.

**Correct Answer:** D. Stavudine (d4T)