Genetic and metabolic disorders: congenital myotonic dystrophy: practice question

A Pediatrics 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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Pediatrics · Genetic and metabolic disorders: congenital myotonic dystrophy

Sample item

A newborn has marked hypotonia, a tented upper lip, bilateral talipes and respiratory distress after a pregnancy complicated by polyhydramnios. His mother has a long thin face and cannot quickly release her handgrip. Why is the infant more severely affected than his mother?

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

The answer and the reasoning

A. Further expansion of a trinucleotide repeat on maternal transmission

Myotonic dystrophy type 1 is caused by a CTG repeat expansion in DMPK. The repeat expands further when transmitted by the mother, causing anticipation and the severe congenital form, with hypotonia, respiratory failure and talipes.

Option by option

  • A. Further expansion of a trinucleotide repeat on maternal transmission (correct)

    Correct. Maternal transmission expands the CTG repeat, giving anticipation and a congenital onset.

  • B. Silencing of the paternal allele by genomic imprinting

    Imprinting explains Prader-Willi and Angelman syndromes, not an affected parent with myotonia.

  • C. A de novo mutation in the infant that his mother lacks

    A de novo mutation would not explain the mother's own features of the same disease.

  • D. Mitochondrial heteroplasmy with a higher mutant load in the infant

    Mitochondrial heteroplasmy explains variable severity in mitochondrial disease and not this repeat disorder with myotonia in the mother.

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 Pediatrics question

What is the answer to this Pediatrics question?

The best answer is A, Further expansion of a trinucleotide repeat on maternal transmission. Myotonic dystrophy type 1 is caused by a CTG repeat expansion in DMPK.

Why is "Silencing of the paternal allele by genomic imprinting" not the answer?

Imprinting explains Prader-Willi and Angelman syndromes, not an affected parent with myotonia.

Why is "A de novo mutation in the infant that his mother lacks" not the answer?

A de novo mutation would not explain the mother's own features of the same disease.

Why is "Mitochondrial heteroplasmy with a higher mutant load in the infant" not the answer?

Mitochondrial heteroplasmy explains variable severity in mitochondrial disease and not this repeat disorder with myotonia in the mother.

What concept does this question test?

It tests Genetic and metabolic disorders: congenital myotonic dystrophy in Pediatrics. The repeat expands further when transmitted by the mother, causing anticipation and the severe congenital form, with hypotonia, respiratory failure and talipes.

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 Genetic and metabolic disorders: congenital myotonic dystrophy?

Memorise milestones and the vaccine schedule as tables and revise them often. Then use the flashcards and the other Pediatrics 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.