Step 1: What is Zidovudine (AZT)?
Zidovudine is an antiretroviral drug used to treat HIV infection. It belongs to the NRTI (Nucleoside Reverse Transcriptase Inhibitor) class. It was the first antiretroviral drug approved by the FDA.
Step 2: HIV replication requires reverse transcriptase.
HIV is a retrovirus. After entering the host cell, its RNA genome must be converted to DNA using an enzyme called reverse transcriptase. This DNA then integrates into the host genome. Blocking reverse transcriptase stops HIV replication.
Step 3: How does Zidovudine work?
Zidovudine is a thymidine analog. Inside the cell it is phosphorylated to its active triphosphate form. This form competes with the natural nucleoside thymidine for incorporation into the growing viral DNA chain. Once incorporated, it causes chain termination because it lacks the 3'-OH group needed for the next nucleotide to attach.
Step 4: Rule out the distractors.
Protease inhibition (Option 1) is the mechanism of drugs like ritonavir and lopinavir. DNA polymerase inhibition (Option 2) is the mechanism of some antivirals like cidofovir, but NOT Zidovudine. Fusion inhibition (Option 4) is the mechanism of enfuvirtide.
Step 5: Confirm the answer.
Zidovudine's mechanism is Nucleoside Reverse Transcriptase Inhibition, specifically chain termination by the triphosphate metabolite.
Answer: Option (3) — Nucleoside reverse transcriptase inhibition