Mutations: reading frame in Duchenne muscular dystrophy: 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 · Mutations: reading frame in Duchenne muscular dystrophy

Sample item

A 4-year-old boy has calf pseudohypertrophy, difficulty rising from the floor and a creatine kinase of 15,000 U/L. Multiplex PCR shows a deletion of several exons of the dystrophin gene. His cousin with a deletion of similar size has a much milder course (Becker dystrophy). What best explains the difference?

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

The answer and the reasoning

A. The boy's deletion is out of frame, giving a premature stop codon and no dystrophin, whereas the cousin's is in frame and gives a shortened functional protein

In the reading-frame rule, deletions that remove a number of nucleotides not divisible by three disrupt the downstream codons and produce a premature stop, leading to nonsense-mediated decay and no dystrophin (Duchenne). In-frame deletions retain both ends of the protein and produce a truncated but partly functional dystrophin (Becker). The gene is X-linked and the largest in the human genome.

Option by option

  • A. The boy's deletion is out of frame, giving a premature stop codon and no dystrophin, whereas the cousin's is in frame and gives a shortened functional protein (correct)

    Frameshift versus in-frame is the explanation for severity.

  • B. The boy's deletion is in an intron and the cousin's is in an exon

    Deletions of exons, not introns, alter the coding sequence, and the described deletions are exonic in both patients.

  • C. The boy has a point mutation in the promoter, whereas the cousin has a triplet repeat expansion

    The scenario does not involve a promoter point mutation or a repeat expansion.

  • D. The boy has a mitochondrial mutation inherited from his father

    The dystrophin gene is on the X chromosome, and mitochondrial inheritance is not involved.

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 A, The boy's deletion is out of frame, giving a premature stop codon and no dystrophin, whereas the cousin's is in frame and gives a shortened functional protein. In the reading-frame rule, deletions that remove a number of nucleotides not divisible by three disrupt the downstream codons and produce a premature stop, leading to nonsense-mediated decay and no dystrophin (Duchenne).

Why is "The boy's deletion is in an intron and the cousin's is in an exon" not the answer?

Deletions of exons, not introns, alter the coding sequence, and the described deletions are exonic in both patients.

Why is "The boy has a point mutation in the promoter, whereas the cousin has…" not the answer?

The scenario does not involve a promoter point mutation or a repeat expansion.

Why is "The boy has a mitochondrial mutation inherited from his father" not the answer?

The dystrophin gene is on the X chromosome, and mitochondrial inheritance is not involved.

What concept does this question test?

It tests Mutations: reading frame in Duchenne muscular dystrophy in Biochemistry. In-frame deletions retain both ends of the protein and produce a truncated but partly functional dystrophin (Becker). The gene is X-linked and the largest in the human genome.

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 Mutations: reading frame in Duchenne muscular dystrophy?

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.