Medical genetics: cystic fibrosis mutation: practice question

A Anatomy 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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Anatomy · Medical genetics: cystic fibrosis mutation

Sample item

A newborn passes no meconium, has abdominal distension and a raised sweat chloride. The most common causative mutation lies on chromosome 7. What is its molecular nature?

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

The answer and the reasoning

D. Three-base deletion removing phenylalanine 508

The commonest CFTR mutation, delta F508, is an in-frame deletion of three nucleotides that removes phenylalanine at position 508. The misfolded protein is degraded and does not reach the cell membrane.

Option by option

  • A. Deletion of the whole gene

    Deletion of the entire gene is rare in cystic fibrosis.

  • B. CGG repeat expansion in the promoter

    CGG repeat expansions cause fragile X syndrome, not cystic fibrosis.

  • C. Nonsense mutation at codon 508

    A nonsense mutation would create a premature stop codon, which is not the common lesion.

  • D. Three-base deletion removing phenylalanine 508 (correct)

    Correct. Delta F508 is an in-frame three-base deletion that causes misfolding and degradation of CFTR.

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 Anatomy question

What is the answer to this Anatomy question?

The best answer is D, Three-base deletion removing phenylalanine 508. The commonest CFTR mutation, delta F508, is an in-frame deletion of three nucleotides that removes phenylalanine at position 508.

Why is "Deletion of the whole gene" not the answer?

Deletion of the entire gene is rare in cystic fibrosis.

Why is "CGG repeat expansion in the promoter" not the answer?

CGG repeat expansions cause fragile X syndrome, not cystic fibrosis.

Why is "Nonsense mutation at codon 508" not the answer?

A nonsense mutation would create a premature stop codon, which is not the common lesion.

What concept does this question test?

It tests Medical genetics: cystic fibrosis mutation in Anatomy. The misfolded protein is degraded and does not reach the cell membrane.

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 Medical genetics: cystic fibrosis mutation?

Learn every nerve by its route, its muscles and what a lesion looks like at the bedside. That one habit answers a large share of questions. Then use the flashcards and the other Anatomy 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.