**Core Concept**
Cobicistat is a potent inhibitor of the cytochrome P450 3A (CYP3A) enzyme, which plays a crucial role in the metabolism of many drugs. By inhibiting CYP3A, cobicistat increases the plasma concentrations of other drugs that are substrates of this enzyme, thereby enhancing their efficacy.

**Why the Correct Answer is Right**
Cobicistat is specifically used in combination with atazanavir, a protease inhibitor, to increase its plasma concentrations and enhance its antiviral efficacy. Atazanavir is a substrate of CYP3A, and cobicistat's inhibition of this enzyme leads to a significant increase in atazanavir's plasma levels. This combination is often used in the treatment of HIV-1 infection.

**Why Each Wrong Option is Incorrect**
**Option A:** Efavirenz is a non-nucleoside reverse transcriptase inhibitor (NNRTI) that does not require boosting with a CYP3A inhibitor like cobicistat.
**Option B:** Ritonavir is another protease inhibitor that is used as a booster, but it is not typically used in combination with cobicistat.
**Option C:** Darunavir is a protease inhibitor that is typically boosted with ritonavir, not cobicistat.

**Clinical Pearl / High-Yield Fact**
Cobicistat is a valuable option for patients who experience gastrointestinal intolerance or other adverse effects associated with ritonavir boosting. Its use can simplify treatment regimens and improve patient adherence to antiretroviral therapy.

**Correct Answer: C. Atazanavir**