Zidovudine and didanosine used in HAART act by :
**Core Concept**
Zidovudine and didanosine are nucleoside reverse transcriptase inhibitors (NRTIs) used in highly active antiretroviral therapy (HAART) for the treatment of HIV infection. These medications work by inhibiting the reverse transcriptase enzyme, which is essential for the replication of retroviruses like HIV.
**Why the Correct Answer is Right**
NRTIs like zidovudine and didanosine act by incorporating themselves into the viral DNA, thereby terminating the viral replication process. They compete with the natural nucleosides for incorporation into the viral DNA, leading to chain termination. This mechanism is specific to the reverse transcriptase enzyme, which is present in retroviruses but not in mammalian cells.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because NRTIs like zidovudine and didanosine do not directly inhibit viral attachment or entry into the host cell.
**Option B:** This option is incorrect because protease inhibitors, not NRTIs, directly inhibit the protease enzyme that is essential for viral maturation.
**Option C:** This option is incorrect because integrase inhibitors, not NRTIs, block the integration of viral DNA into the host genome.
**Clinical Pearl / High-Yield Fact**
NRTIs like zidovudine and didanosine can cause mitochondrial toxicity, leading to bone marrow suppression and lactic acidosis. It is essential to monitor patients for these side effects, especially when initiating therapy with these medications.
**Correct Answer:** C. They inhibit the reverse transcriptase enzyme.