Vesiculobullous disorders: azathioprine and TPMT: practice question

A Dermatology 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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Dermatology · Vesiculobullous disorders: azathioprine and TPMT

Sample item

A patient with pemphigus vulgaris is to start azathioprine as a steroid-sparing agent. Which test should be done first to reduce the risk of severe myelosuppression?

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

The answer and the reasoning

D. Thiopurine methyltransferase activity

Low or absent TPMT activity leads to accumulation of toxic thioguanine nucleotides from azathioprine and severe bone marrow suppression. Testing before starting allows dose reduction or an alternative drug.

Option by option

  • A. Serum G6PD

    G6PD is tested before dapsone or primaquine for haemolysis, not before azathioprine.

  • B. HLA-B*5801

    HLA-B*5801 predicts allopurinol hypersensitivity.

  • C. HLA-B*1502

    HLA-B*1502 predicts carbamazepine-induced SJS and TEN.

  • D. Thiopurine methyltransferase activity (correct)

    Correct. Reduced TPMT activity predisposes to azathioprine-induced marrow toxicity.

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.

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Questions about this Dermatology question

What is the answer to this Dermatology question?

The best answer is D, Thiopurine methyltransferase activity. Low or absent TPMT activity leads to accumulation of toxic thioguanine nucleotides from azathioprine and severe bone marrow suppression.

Why is "Serum G6PD" not the answer?

G6PD is tested before dapsone or primaquine for haemolysis, not before azathioprine.

Why is "HLA-B*5801" not the answer?

HLA-B*5801 predicts allopurinol hypersensitivity.

Why is "HLA-B*1502" not the answer?

HLA-B*1502 predicts carbamazepine-induced SJS and TEN.

What concept does this question test?

It tests Vesiculobullous disorders: azathioprine and TPMT in Dermatology. Testing before starting allows dose reduction or an alternative drug.

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 Vesiculobullous disorders: azathioprine and TPMT?

Learn each condition by its primary lesion, site and signature sign. Then use the flashcards and the other Dermatology 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.