Papulosquamous disorders: mechanism of PUVA: 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 · Papulosquamous disorders: mechanism of PUVA

Sample item

In PUVA therapy for psoriasis, what is the molecular action of the oral psoralen once the skin is exposed to UVA?

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

The answer and the reasoning

D. Forms cross-links between DNA strands, inhibiting keratinocyte proliferation

Psoralen intercalates into DNA and, after UVA photoactivation, forms covalent cross-links between pyrimidine bases of opposite strands. This slows hyperproliferation and also alters the immune response. Long-term use raises the risk of skin cancer.

Option by option

  • A. Binds the vitamin D receptor on keratinocytes

    Binding to the vitamin D receptor describes calcipotriol.

  • B. Blocks dihydrofolate reductase in epidermal cells

    Blocking dihydrofolate reductase is the action of methotrexate.

  • C. Inhibits phosphodiesterase 4 in inflammatory cells

    PDE4 inhibition describes apremilast.

  • D. Forms cross-links between DNA strands, inhibiting keratinocyte proliferation (correct)

    Correct. Photoactivated psoralen cross-links DNA and reduces keratinocyte proliferation.

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, Forms cross-links between DNA strands, inhibiting keratinocyte proliferation. Psoralen intercalates into DNA and, after UVA photoactivation, forms covalent cross-links between pyrimidine bases of opposite strands.

Why is "Binds the vitamin D receptor on keratinocytes" not the answer?

Binding to the vitamin D receptor describes calcipotriol.

Why is "Blocks dihydrofolate reductase in epidermal cells" not the answer?

Blocking dihydrofolate reductase is the action of methotrexate.

Why is "Inhibits phosphodiesterase 4 in inflammatory cells" not the answer?

PDE4 inhibition describes apremilast.

What concept does this question test?

It tests Papulosquamous disorders: mechanism of PUVA in Dermatology. This slows hyperproliferation and also alters the immune response. Long-term use raises the risk of skin cancer.

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 Papulosquamous disorders: mechanism of PUVA?

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.