Which of the following drugs is a protease inhibitor used in HIV management?

The correct answer is ritonavir, which is classified as a protease inhibitor and is significantly used in the management of HIV. Protease inhibitors work by inhibiting the protease enzyme, which is essential for the HIV virus to replicate. By blocking this enzyme, ritonavir prevents the maturation of viral particles, thus reducing the viral load in the patient's system.

Ritonavir is often used in combination with other antiretroviral drugs to enhance their efficacy. Its role is crucial in various antiretroviral therapy regimens, especially because it can also enhance the levels of other protease inhibitors when used at low doses.

The other options listed are not protease inhibitors; they belong to different classes of antiretroviral medications. Raltegravir is an integrase inhibitor, efavirenz is a non-nucleoside reverse transcriptase inhibitor (NNRTI), and lamivudine is a nucleoside reverse transcriptase inhibitor (NRTI). Each of these has a distinct mechanism of action, which differs from that of protease inhibitors like ritonavir.