**Core Concept**
Atazanavir is a protease inhibitor used in the treatment of HIV-1 infection. Protease inhibitors work by binding to the protease enzyme, preventing the maturation of viral particles, and ultimately inhibiting viral replication.

**Why the Correct Answer is Right**
The correct answer states that resistance to atazanavir is due to a mutation in codon 50, where isoleucine is substituted with valine. This mutation affects the protease enzyme, allowing the virus to evade the inhibitory effects of atazanavir. This is a well-documented mechanism of resistance to atazanavir, and it highlights the importance of monitoring for resistance mutations in patients on protease inhibitor therapy.

* **Option A:** This statement is incorrect because atazanavir is effective against both HIV-1 and HIV-2, although its efficacy against HIV-2 is less well established.
* **Option C:** This statement is incorrect because atazanavir can actually increase levels of cholesterol and triglycerides, which can contribute to cardiovascular risk in patients on antiretroviral therapy.
* **Option D:** This statement is incorrect because the combination of atazanavir with ritonavir (a pharmacokinetic enhancer) does have an advantage in terms of increasing atazanavir levels and improving its efficacy.

**Clinical Pearl / High-Yield Fact**
Resistance testing is an important tool in the management of HIV-1 infection, allowing clinicians to identify mutations associated with resistance to specific antiretroviral agents. This information can inform treatment decisions and help prevent the development of cross-resistance between different classes of antiretroviral agents.

✓ Correct Answer: B. Resistance is due to mutation in codon 50 isoleucine to valine substitution