**Core Concept**
The question revolves around the concept of active metabolites in protease inhibitors, which are a class of antiretroviral medications used to treat HIV/AIDS. Protease inhibitors work by binding to the protease enzyme, preventing it from cleaving viral proteins, thereby inhibiting viral replication.

**Why the Correct Answer is Right**
Atazanavir, a protease inhibitor, undergoes extensive metabolism, primarily through the cytochrome P450 3A4 (CYP3A4) enzyme, resulting in the formation of an active metabolite, M8. The M8 metabolite has been shown to contribute significantly to the antiviral activity of atazanavir, making it a key component of the drug's mechanism of action.

**Why Each Wrong Option is Incorrect**
* **Option A:** Lopinavir does not have a significant active metabolite. It is primarily metabolized by CYP3A4, but its main metabolites are inactive.
* **Option B:** Darunavir is primarily metabolized by CYP3A4, but its main metabolites are also inactive. While it does have some minor active metabolites, they are not as significant as the active metabolite of atazanavir.
* **Option D:** Indinavir is primarily metabolized by CYP3A4, resulting in the formation of several inactive metabolites. It does not have a significant active metabolite.

**Clinical Pearl / High-Yield Fact**
When prescribing protease inhibitors, it's essential to consider the potential for drug interactions with other medications that may affect CYP3A4 activity, as this can impact the efficacy and safety of the protease inhibitor.

**Correct Answer: C. Atazanavir**