**Core Concept**
Fusion inhibitors are a class of antiretroviral drugs used in the treatment of HIV/AIDS. They work by preventing the fusion of the HIV virus with the host cell membrane, thereby blocking the entry of the virus into the cell.

**Why the Correct Answer is Right**
Enfuvirtide (also known as T-20) is a synthetic peptide that mimics a region of the HIV envelope glycoprotein gp41. By binding to gp41, enfuvirtide prevents the conformational change required for the fusion of the HIV virus with the host cell membrane. This action inhibits the entry of the virus into the cell, thereby reducing the viral load and slowing disease progression. Enfuvirtide is administered via subcutaneous injection and is used in combination with other antiretroviral medications.

**Why Each Wrong Option is Incorrect**
**Option B:** Ritonavir is a protease inhibitor, not a fusion inhibitor. It works by binding to the HIV protease enzyme and preventing the maturation of viral particles.
**Option C:** Efavirenz is a non-nucleoside reverse transcriptase inhibitor (NNRTI), which binds to the HIV reverse transcriptase enzyme and prevents viral replication.
**Option D:** Didanosine is a nucleoside reverse transcriptase inhibitor (NRTI), which is incorporated into the viral DNA and terminates its elongation.

**Clinical Pearl / High-Yield Fact**
Fusion inhibitors like enfuvirtide are particularly useful in patients with HIV who have failed other antiretroviral therapies due to resistance mutations.

**✓ Correct Answer: A. Enfuvirtide**