**Core Concept**
Monitoring HIV disease progression involves assessing both the immune system's integrity and the virus's replication activity. CD4+ T-cell counts reflect the host's immune response, while plasma HIV RNA levels indicate viral load and replication activity.

**Why the Correct Answer is Right**
The correct answer combines both CD4+ T-cell counts and plasma HIV RNA levels. CD4+ T-cell counts provide a quantitative measure of the host's immune response, while plasma HIV RNA levels offer a direct assessment of viral replication activity. This dual approach allows for a comprehensive understanding of HIV disease progression. The CD4+ T-cell count is used to monitor the host's immune system, and a decrease in count is indicative of immune system damage. Conversely, plasma HIV RNA levels measure the amount of virus present in the blood, with higher levels indicating active replication. By considering both parameters, healthcare providers can better assess disease progression and make informed treatment decisions.

**Why Each Wrong Option is Incorrect**
**Option A:** Focusing solely on CD4+ T-cell counts may not provide a complete picture of disease progression, as it does not account for viral replication activity. This approach may lead to undertreatment or delayed treatment initiation.
**Option B:** Relying solely on plasma HIV RNA levels may not account for the host's immune response and potential immune system damage. This approach may lead to overestimation of treatment efficacy or underestimation of disease progression.
**Option D:** This option is incorrect because it suggests an either-or approach, implying that either CD4+ T-cell counts or plasma HIV RNA levels are sufficient for monitoring disease progression. In reality, both parameters are essential for a comprehensive understanding of HIV disease progression.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that regular monitoring of both CD4+ T-cell counts and plasma HIV RNA levels is critical in managing HIV infection. This dual approach allows for timely intervention and adjustment of antiretroviral therapy to maintain viral suppression and prevent disease progression.

**✓ Correct Answer: C. Peripheral blood CD4 + Tcell counts with Plasma levels of HIV RNA.