DNA repair: Cockayne syndrome: practice question

A Biochemistry 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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Biochemistry · DNA repair: Cockayne syndrome

Sample item

A 4-year-old has growth failure, microcephaly, sensorineural deafness, retinopathy and severe sunburn on minimal exposure, but there are no skin cancers. Cultured cells fail to resume RNA synthesis after UV damage. Which repair process is defective?

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

The answer and the reasoning

C. Transcription-coupled nucleotide excision repair

In Cockayne syndrome (CSA or CSB mutations), lesions that block RNA polymerase cannot be repaired preferentially in transcribed strands. Cells show failure of RNA synthesis recovery after UV, giving neurodegeneration and premature ageing, while skin cancer risk is not increased. Defects in global genome repair, as in xeroderma pigmentosum group C, instead cause multiple skin cancers.

Option by option

  • A. Global genome nucleotide excision repair

    Global genome repair defects cause xeroderma pigmentosum with early, multiple skin cancers.

  • B. Mismatch repair

    Mismatch repair defects cause microsatellite instability and colorectal cancer rather than photosensitivity.

  • C. Transcription-coupled nucleotide excision repair (correct)

    Failure of transcription-coupled repair explains the neurodegeneration, photosensitivity and absent skin cancers.

  • D. Non-homologous end joining

    NHEJ defects cause radiosensitive immunodeficiency and do not produce UV-related failure to recover RNA synthesis.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, Transcription-coupled nucleotide excision repair. In Cockayne syndrome (CSA or CSB mutations), lesions that block RNA polymerase cannot be repaired preferentially in transcribed strands.

Why is "Global genome nucleotide excision repair" not the answer?

Global genome repair defects cause xeroderma pigmentosum with early, multiple skin cancers.

Why is "Mismatch repair" not the answer?

Mismatch repair defects cause microsatellite instability and colorectal cancer rather than photosensitivity.

Why is "Non-homologous end joining" not the answer?

NHEJ defects cause radiosensitive immunodeficiency and do not produce UV-related failure to recover RNA synthesis.

What concept does this question test?

It tests DNA repair: Cockayne syndrome in Biochemistry. Cells show failure of RNA synthesis recovery after UV, giving neurodegeneration and premature ageing, while skin cancer risk is not increased. Defects in global genome repair, as in xeroderma pigmentosum group C, instead cause multiple skin cancers.

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 DNA repair: Cockayne syndrome?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.