**Core Concept**
The combination of ritonavir with lopinavir is based on the principle of pharmacokinetic boosting, where ritonavir inhibits the cytochrome P450 3A4 enzyme, thereby increasing the plasma levels of lopinavir. This is an example of pharmacokinetic drug interaction.

**Why the Correct Answer is Right**
Ritonavir is a potent inhibitor of the cytochrome P450 3A4 enzyme, which is responsible for the metabolism of lopinavir. By inhibiting this enzyme, ritonavir reduces the metabolism of lopinavir, leading to increased plasma levels and a prolongation of its half-life. This allows for a once-daily dosing regimen of lopinavir/ritonavir, making it more convenient for patients to adhere to their treatment regimen.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because ritonavir does not directly inhibit the activity of lopinavir. Instead, it inhibits the enzyme responsible for metabolizing lopinavir.
**Option B:** This is incorrect because ritonavir does not enhance the activity of lopinavir. It simply increases its plasma levels by inhibiting its metabolism.
**Option C:** This is incorrect because ritonavir does not have a synergistic effect with lopinavir. It works by inhibiting the enzyme responsible for metabolizing lopinavir.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that ritonavir is not just used as a booster for lopinavir, but also for other protease inhibitors like atazanavir and darunavir. This knowledge can be useful in clinical practice when managing HIV-infected patients.

**Correct Answer: C. Ritonavir inhibits the cytochrome P450 3A4 enzyme, thereby increasing the plasma levels of lopinavir.**