HIV virology fundamentals: Which unique feature best explains why HIV is classified as a retrovirus and is considered distinctive among human pathogens?

Introduction / Context:
HIV is a member of the Retroviridae. The hallmark of retroviruses is their replication strategy, which requires copying RNA into DNA inside the host cell. Identifying the signature enzyme clarifies both the name ‘‘retro’’ and many drug targets.

Given Data / Assumptions:

• HIV carries two copies of single-stranded RNA (diploid RNA genome).
• Reverse transcription converts RNA to DNA in the host cytoplasm.
• The question seeks the unique feature underlying HIV’s retroviral identity.

Concept / Approach:
Reverse transcriptase is the defining enzyme of retroviruses. It catalyzes RNA-dependent DNA polymerization, DNA-dependent DNA polymerization, and RNase H activity. This mechanism differentiates retroviruses from most other RNA viruses and explains susceptibility to nucleoside and non-nucleoside reverse transcriptase inhibitors.

Step-by-Step Solution:
Identify retroviral hallmark → reverse transcriptase.Exclude incorrect claims such as mixed DNA/RNA genomes within the same virion.Note that HIV is enveloped and has specific glycoproteins; the unique trait among options is the enzyme.Choose ‘‘a viral enzyme known as reverse transcriptase’’.

Verification / Alternative check:
Antiretroviral drug classes directly target reverse transcriptase, underlining its centrality.

Why Other Options Are Wrong:

• More than 100 copies of the genome: HIV carries two RNA copies, not >100.
• Both DNA and RNA genome: Incorrect; the mature virion contains RNA.
• Sticky proteins: gp120/gp41 mediate binding/fusion but are not unique to retroviruses.
• Icosahedral capsid without envelope: HIV is enveloped; this is false.

Common Pitfalls:
Confusing the presence of both nucleic acids within infected cells (proviral DNA) with virion composition.

Final Answer:
a viral enzyme known as reverse transcriptase.