Muscle pathology: Duchenne and Becker dystrophy: practice question

A Pathology 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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Pathology · Muscle pathology: Duchenne and Becker dystrophy

Sample item

A 5-year-old boy has calf pseudohypertrophy, a Gowers sign and creatine kinase of 15,000 U/L. His muscle biopsy shows absent dystrophin staining. Why does Becker muscular dystrophy, caused by mutation of the same gene, run a milder course?

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

The answer and the reasoning

B. The mutations preserve the reading frame, so a truncated, partly functional dystrophin is made

Duchenne dystrophy usually results from frameshift deletions in the large DMD gene at Xp21 that produce no functional dystrophin, so the muscle membrane is fragile and fibres undergo repeated necrosis, replaced by fat and fibrous tissue. In Becker dystrophy the deletions keep the reading frame, so a shortened but partially functional protein remains. This is called the reading-frame rule.

Option by option

  • A. The mutations are autosomal and are compensated by the second allele

    The disease is X-linked recessive, with no second allele in affected males.

  • B. The mutations preserve the reading frame, so a truncated, partly functional dystrophin is made (correct)

    Correct. In-frame mutations allow a partly functional protein, giving milder disease.

  • C. The mutations cause overproduction of dystrophin in skeletal muscle

    Dystrophin is reduced or abnormal in Becker disease and not overproduced.

  • D. The mutations lie in mitochondrial DNA and are inherited maternally

    The DMD gene lies on the X chromosome, not in mitochondrial DNA.

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 Pathology question

What is the answer to this Pathology question?

The best answer is B, The mutations preserve the reading frame, so a truncated, partly functional dystrophin is made. Duchenne dystrophy usually results from frameshift deletions in the large DMD gene at Xp21 that produce no functional dystrophin, so the muscle membrane is fragile and fibres undergo repeated necrosis, replaced by fat and fibrous tissue.

Why is "The mutations are autosomal and are compensated by the second allele" not the answer?

The disease is X-linked recessive, with no second allele in affected males.

Why is "The mutations cause overproduction of dystrophin in skeletal muscle" not the answer?

Dystrophin is reduced or abnormal in Becker disease and not overproduced.

Why is "The mutations lie in mitochondrial DNA and are inherited maternally" not the answer?

The DMD gene lies on the X chromosome, not in mitochondrial DNA.

What concept does this question test?

It tests Muscle pathology: Duchenne and Becker dystrophy in Pathology. In Becker dystrophy the deletions keep the reading frame, so a shortened but partially functional protein remains. This is called 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 Muscle pathology: Duchenne and Becker dystrophy?

Learn each disease as a bundle: cause, key histology, markers, and the clinical hallmark. Then use the flashcards and the other Pathology 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.