Drug interactions: practice question

A Pharmacology 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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Pharmacology · Drug interactions

Sample item

A kidney transplant patient on azathioprine is started on allopurinol for gout and develops pancytopenia. What explains this?

AAllopurinol induces hepatic enzymes producing toxic metabolitesBAllopurinol displaces azathioprine from plasma proteinsCXanthine oxidase inhibition prevents inactivation of 6-mercaptopurineDAllopurinol blocks the conversion of azathioprine to its active form

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

The answer and the reasoning

C. Xanthine oxidase inhibition prevents inactivation of 6-mercaptopurine

Azathioprine is converted to 6-mercaptopurine, which xanthine oxidase inactivates to thiouric acid. Allopurinol blocks this enzyme, raising active thiopurine levels and causing marrow toxicity, so the azathioprine dose must be greatly reduced.

Option by option

A. Allopurinol induces hepatic enzymes producing toxic metabolites
Allopurinol inhibits rather than induces enzymes.
B. Allopurinol displaces azathioprine from plasma proteins
Protein-binding displacement is not the mechanism of this interaction.
C. Xanthine oxidase inhibition prevents inactivation of 6-mercaptopurine (correct)
Blocking xanthine oxidase leaves more active thiopurine, causing marrow toxicity.
D. Allopurinol blocks the conversion of azathioprine to its active form
Activation of azathioprine is not impaired; active drug accumulates instead.

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 Pharmacology question

What is the answer to this Pharmacology question?

The best answer is C, Xanthine oxidase inhibition prevents inactivation of 6-mercaptopurine. Azathioprine is converted to 6-mercaptopurine, which xanthine oxidase inactivates to thiouric acid.

Why is "Allopurinol induces hepatic enzymes producing toxic metabolites" not the answer?

Allopurinol inhibits rather than induces enzymes.

Why is "Allopurinol displaces azathioprine from plasma proteins" not the answer?

Protein-binding displacement is not the mechanism of this interaction.

Why is "Allopurinol blocks the conversion of azathioprine to its active form" not the answer?

Activation of azathioprine is not impaired; active drug accumulates instead.

What concept does this question test?

It tests Drug interactions in Pharmacology. Allopurinol blocks this enzyme, raising active thiopurine levels and causing marrow toxicity, so the azathioprine dose must be greatly reduced.

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 Drug interactions?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology 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.