Genetics: 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 · Genetics

Sample item

A 7-year-old boy has intellectual disability, a long face, large ears, a prominent jaw and large testes. His maternal uncle has a similar problem. The underlying genetic defect is:

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

The answer and the reasoning

B. CGG trinucleotide repeat expansion in the FMR1 gene

The facial features, macro-orchidism and X-linked pattern indicate Fragile X syndrome, the commonest inherited cause of intellectual disability. It results from an expanded CGG repeat in FMR1 with hypermethylation and silencing of the gene.

Option by option

  • A. CAG repeat expansion in the huntingtin gene

    CAG expansion causes Huntington disease, which is autosomal dominant and presents with chorea.

  • B. CGG trinucleotide repeat expansion in the FMR1 gene (correct)

    Correct. Expanded CGG repeats in FMR1 cause Fragile X syndrome.

  • C. CTG repeat expansion in the DMPK gene

    CTG expansion causes myotonic dystrophy with myotonia and weakness.

  • D. GAA repeat expansion in the frataxin gene

    GAA expansion causes Friedreich ataxia, an autosomal recessive disease.

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 B, CGG trinucleotide repeat expansion in the FMR1 gene. The facial features, macro-orchidism and X-linked pattern indicate Fragile X syndrome, the commonest inherited cause of intellectual disability.

Why is "CAG repeat expansion in the huntingtin gene" not the answer?

CAG expansion causes Huntington disease, which is autosomal dominant and presents with chorea.

Why is "CTG repeat expansion in the DMPK gene" not the answer?

CTG expansion causes myotonic dystrophy with myotonia and weakness.

Why is "GAA repeat expansion in the frataxin gene" not the answer?

GAA expansion causes Friedreich ataxia, an autosomal recessive disease.

What concept does this question test?

It tests Genetics in Pediatrics. It results from an expanded CGG repeat in FMR1 with hypermethylation and silencing of the gene.

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 Genetics?

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.