**Core Concept**
Zidovudine (AZT) is a nucleoside reverse transcriptase inhibitor (NRTI) used in the treatment of HIV infection. The selective toxicity of zidovudine is based on its mechanism of action, which involves the inhibition of viral replication by targeting a specific enzyme essential for the viral life cycle.

**Why the Correct Answer is Right**
Zidovudine selectively inhibits the reverse transcriptase enzyme of HIV, which is responsible for converting viral RNA into DNA, allowing the virus to integrate into the host cell's genome. This enzyme is not present in human cells, making zidovudine's action selective for the virus. The correct answer is reverse transcriptase.

**Why Each Wrong Option is Incorrect**
**Option A:** DNA polymerase - This enzyme is present in human cells and is not the primary target of zidovudine. While DNA polymerase is involved in DNA replication, it is not the enzyme responsible for the selective toxicity of zidovudine.

**Option B:** Protease - This enzyme is also involved in the HIV life cycle, but it is not the primary target of zidovudine. Protease inhibitors are a different class of antiretroviral drugs that target a different enzyme.

**Option C:** Integrase - This enzyme is involved in the integration of viral DNA into the host cell's genome, but it is not the primary target of zidovudine. Integrase inhibitors are a different class of antiretroviral drugs that target a different enzyme.

**Clinical Pearl / High-Yield Fact**
NRTIs like zidovudine work by incorporating themselves into the viral DNA strand, causing chain termination and preventing viral replication. This mechanism of action is unique to NRTIs and is the basis for their selective toxicity.

**Correct Answer: A. Reverse transcriptase.**