Genetics: dystrophinopathies: 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 · Genetics: dystrophinopathies

Sample item

A 14-year-old boy has mild proximal weakness and calf pseudohypertrophy and still walks independently. Muscle biopsy shows reduced truncated dystrophin. His cousin with Duchenne muscular dystrophy lost the ability to walk at 11. Which molecular mechanism best explains the milder course?

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

The answer and the reasoning

B. In-frame deletion preserving the reading frame, giving partly functional truncated dystrophin

Both conditions arise from mutations in the X-linked dystrophin gene. In Duchenne muscular dystrophy, frameshift mutations (often exon deletions) cause absence of dystrophin. In Becker muscular dystrophy, in-frame mutations produce a shorter protein that retains some function, hence later onset and a slower course. This is the reading-frame rule.

Option by option

  • A. Sarcoglycan gene mutation with autosomal recessive inheritance

    Sarcoglycanopathies are limb-girdle dystrophies, and the protein reduced here is dystrophin.

  • B. In-frame deletion preserving the reading frame, giving partly functional truncated dystrophin (correct)

    Correct. An in-frame deletion yields a shortened but partly functional protein, giving Becker dystrophy.

  • C. Frameshift deletion causing a premature stop codon and no dystrophin

    A frameshift with no protein is the mechanism of the severe Duchenne phenotype.

  • D. Trinucleotide repeat expansion in the dystrophin promoter

    Dystrophinopathies are not caused by promoter repeat expansion; that is a mechanism in fragile X syndrome.

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 B, In-frame deletion preserving the reading frame, giving partly functional truncated dystrophin. Both conditions arise from mutations in the X-linked dystrophin gene.

Why is "Sarcoglycan gene mutation with autosomal recessive inheritance" not the answer?

Sarcoglycanopathies are limb-girdle dystrophies, and the protein reduced here is dystrophin.

Why is "Frameshift deletion causing a premature stop codon and no dystrophin" not the answer?

A frameshift with no protein is the mechanism of the severe Duchenne phenotype.

Why is "Trinucleotide repeat expansion in the dystrophin promoter" not the answer?

Dystrophinopathies are not caused by promoter repeat expansion; that is a mechanism in fragile X syndrome.

What concept does this question test?

It tests Genetics: dystrophinopathies in Anatomy. In Duchenne muscular dystrophy, frameshift mutations (often exon deletions) cause absence of dystrophin. In Becker muscular dystrophy, in-frame mutations produce a shorter protein that retains some function, hence later onset and a slower course. This is the reading-frame rule.

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 Genetics: dystrophinopathies?

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.