**Core Concept**
The question is testing the concept of pharmacokinetic interactions between protease inhibitors and other antiretroviral agents in the treatment of HIV/AIDS. Specifically, it's inquiring about the mechanism of pharmacokinetic boosting, which enhances the plasma concentrations of protease inhibitors.

**Why the Correct Answer is Right**
Cobicistat, a potent inhibitor of the cytochrome P450 3A4 enzyme, is used as a pharmacokinetic booster to increase the plasma concentrations of atazanavir, darunavir, and other protease inhibitors. This boosting effect allows for once-daily dosing of these protease inhibitors, improving patient adherence and treatment outcomes.

**Why Each Wrong Option is Incorrect**
**Option A:** Ritonavir, another protease inhibitor, is not typically used as a booster, although it does have some inhibitory effect on CYP3A4.
**Option B:** Lopinavir, a protease inhibitor itself, is not a booster.
**Option C:** Atazanavir, a protease inhibitor, does not boost other protease inhibitors.

**Clinical Pearl / High-Yield Fact**
When using a protease inhibitor with a pharmacokinetic booster, it's essential to consider potential drug-drug interactions, particularly with other medications metabolized by CYP3A4.

**Correct Answer:** C. Cobicistat.