Virology: selectivity of acyclovir: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Microbiology · Virology: selectivity of acyclovir

Sample item

Acyclovir is much less toxic to uninfected cells than to cells infected with herpes simplex virus. What is the main reason?

AIt blocks viral entry by binding envelope glycoproteinsBIt inhibits a viral neuraminidase absent from host cellsCIt is concentrated only in sensory gangliaDInitial phosphorylation needs viral thymidine kinase

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. Initial phosphorylation needs viral thymidine kinase

Acyclovir is a guanosine analogue that is monophosphorylated only by herpesvirus thymidine kinase, then converted by cellular kinases to the triphosphate. This form inhibits viral DNA polymerase and terminates the chain, so uninfected cells generate little active drug.

Option by option

A. It blocks viral entry by binding envelope glycoproteins
Acyclovir acts on viral DNA synthesis and does not block viral entry.
B. It inhibits a viral neuraminidase absent from host cells
Herpesviruses lack neuraminidase, which is the target of oseltamivir in influenza.
C. It is concentrated only in sensory ganglia
Acyclovir is distributed widely and is not restricted to ganglia.
D. Initial phosphorylation needs viral thymidine kinase (correct)
Correct. Viral thymidine kinase makes the first activation step, restricting activation to infected cells.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is D, Initial phosphorylation needs viral thymidine kinase. Acyclovir is a guanosine analogue that is monophosphorylated only by herpesvirus thymidine kinase, then converted by cellular kinases to the triphosphate.

Why is "It blocks viral entry by binding envelope glycoproteins" not the answer?

Acyclovir acts on viral DNA synthesis and does not block viral entry.

Why is "It inhibits a viral neuraminidase absent from host cells" not the answer?

Herpesviruses lack neuraminidase, which is the target of oseltamivir in influenza.

Why is "It is concentrated only in sensory ganglia" not the answer?

Acyclovir is distributed widely and is not restricted to ganglia.

What concept does this question test?

It tests Virology: selectivity of acyclovir in Microbiology. This form inhibits viral DNA polymerase and terminates the chain, so uninfected cells generate little active drug.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Virology: selectivity of acyclovir?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.