Genetics: fragile X: 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: fragile X

Sample item

A boy has intellectual disability, long face, large ears and macro-orchidism after puberty. Which genetic abnormality is responsible?

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

Fragile X syndrome is an X-linked disorder from a CGG repeat expansion (more than 200) in FMR1, causing methylation and silencing. It is the commonest inherited cause of intellectual disability and shows anticipation. Autism features and mitral valve prolapse can occur.

Option by option

  • A. Trisomy 21

    Trisomy 21 has a different phenotype.

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

    Correct. FMR1 CGG expansion causes fragile X.

  • C. CAG repeat in the HTT gene

    CAG repeats cause Huntington disease.

  • D. Deletion of 15q11-13

    15q11-13 deletion causes Prader-Willi or Angelman 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 Pediatrics question

What is the answer to this Pediatrics question?

The best answer is B, CGG trinucleotide repeat expansion in the FMR1 gene. Fragile X syndrome is an X-linked disorder from a CGG repeat expansion (more than 200) in FMR1, causing methylation and silencing.

Why is "Trisomy 21" not the answer?

Trisomy 21 has a different phenotype.

Why is "CAG repeat in the HTT gene" not the answer?

CAG repeats cause Huntington disease.

Why is "Deletion of 15q11-13" not the answer?

15q11-13 deletion causes Prader-Willi or Angelman syndrome.

What concept does this question test?

It tests Genetics: fragile X in Pediatrics. It is the commonest inherited cause of intellectual disability and shows anticipation. Autism features and mitral valve prolapse can occur.

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: fragile X?

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.