Virology: antiviral resistance: 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: antiviral resistance

Sample item

A man with AIDS has chronic perianal herpes simplex ulcers that do not heal despite 2 weeks of intravenous acyclovir. The isolate lacks viral thymidine kinase activity. Which drug is most appropriate?

AValacyclovirBFoscarnetCFamciclovirDOral acyclovir at a higher dose

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

The answer and the reasoning

B. Foscarnet

Acyclovir needs phosphorylation by viral thymidine kinase, and most acyclovir-resistant herpes simplex isolates have thymidine kinase mutations. Foscarnet is a pyrophosphate analogue that directly inhibits viral DNA polymerase without needing activation, so it remains active. Its main toxicities are nephrotoxicity and electrolyte disturbance.

Option by option

A. Valacyclovir
Valacyclovir is a prodrug of acyclovir and still depends on viral thymidine kinase.
B. Foscarnet (correct)
Correct. Foscarnet acts directly on viral DNA polymerase and bypasses thymidine kinase.
C. Famciclovir
Famciclovir becomes penciclovir, which also needs viral thymidine kinase for activation.
D. Oral acyclovir at a higher dose
Higher doses of acyclovir will not work if the drug cannot be activated.

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 B, Foscarnet. Acyclovir needs phosphorylation by viral thymidine kinase, and most acyclovir-resistant herpes simplex isolates have thymidine kinase mutations.

Why is "Valacyclovir" not the answer?

Valacyclovir is a prodrug of acyclovir and still depends on viral thymidine kinase.

Why is "Famciclovir" not the answer?

Famciclovir becomes penciclovir, which also needs viral thymidine kinase for activation.

Why is "Oral acyclovir at a higher dose" not the answer?

Higher doses of acyclovir will not work if the drug cannot be activated.

What concept does this question test?

It tests Virology: antiviral resistance in Microbiology. Foscarnet is a pyrophosphate analogue that directly inhibits viral DNA polymerase without needing activation, so it remains active. Its main toxicities are nephrotoxicity and electrolyte disturbance.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: antiviral resistance?

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.