**Core Concept**
Reverse transcriptase inhibitors (RTIs) are a class of antiretroviral drugs used to treat HIV infection. They work by inhibiting the activity of reverse transcriptase, an enzyme essential for the replication of retroviruses like HIV. This mechanism prevents the virus from converting its RNA genome into DNA, thereby blocking viral replication.

**Why the Correct Answer is Right**
Abacavir is a nucleoside reverse transcriptase inhibitor (NRTI) that specifically targets the HIV reverse transcriptase enzyme. It acts by incorporating itself into the viral DNA, resulting in chain termination and inhibition of viral replication. This is achieved through the incorporation of a false nucleotide, which prevents the addition of additional nucleotides and halts the viral replication process.

**Why Each Wrong Option is Incorrect**
**Option A:** Indinavir is a protease inhibitor, not a reverse transcriptase inhibitor. It targets the HIV protease enzyme, preventing the maturation of viral particles.

**Option B:** Ritonavir is also a protease inhibitor, used in combination with other antiretroviral drugs to enhance their efficacy. It is not a reverse transcriptase inhibitor.

**Option C:** Nelfinavir is another protease inhibitor, similar to indinavir and ritonavir. It is not a reverse transcriptase inhibitor.

**Clinical Pearl / High-Yield Fact**
When prescribing antiretroviral therapy, it's essential to consider the potential for drug interactions and resistance development. Reverse transcriptase inhibitors like abacavir can interact with other medications, including certain antacids and herbal supplements, which may impact their efficacy or safety.

**✓ Correct Answer: D. Abacavir**