Child and adolescent psychiatry: fragile X syndrome: practice question

A Psychiatry 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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Psychiatry · Child and adolescent psychiatry: fragile X syndrome

Sample item

A 6-year-old boy has intellectual disability, a long face, large ears, flat feet and hand-flapping with poor eye contact. His maternal uncle has similar problems. What is the underlying genetic abnormality?

ACAG repeat expansion in the huntingtin geneBLoss-of-function mutation in the MECP2 geneCGAA repeat expansion in the frataxin geneDCGG repeat expansion in the FMR1 gene on the X chromosome

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

The answer and the reasoning

D. CGG repeat expansion in the FMR1 gene on the X chromosome

Fragile X syndrome is caused by a CGG trinucleotide expansion in FMR1, with methylation and silencing of the gene when the repeat exceeds 200. It is the most common inherited cause of intellectual disability and is associated with autism features and macro-orchidism after puberty.

Option by option

A. CAG repeat expansion in the huntingtin gene
A CAG repeat in huntingtin causes Huntington disease.
B. Loss-of-function mutation in the MECP2 gene
MECP2 mutation causes Rett syndrome, mainly in girls.
C. GAA repeat expansion in the frataxin gene
A GAA repeat in frataxin causes Friedreich ataxia.
D. CGG repeat expansion in the FMR1 gene on the X chromosome (correct)
Correct. A CGG expansion silences FMR1 and causes fragile X.

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.

Questions about this Psychiatry question

What is the answer to this Psychiatry question?

The best answer is D, CGG repeat expansion in the FMR1 gene on the X chromosome. Fragile X syndrome is caused by a CGG trinucleotide expansion in FMR1, with methylation and silencing of the gene when the repeat exceeds 200.

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

A CAG repeat in huntingtin causes Huntington disease.

Why is "Loss-of-function mutation in the MECP2 gene" not the answer?

MECP2 mutation causes Rett syndrome, mainly in girls.

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

A GAA repeat in frataxin causes Friedreich ataxia.

What concept does this question test?

It tests Child and adolescent psychiatry: fragile X syndrome in Psychiatry. It is the most common inherited cause of intellectual disability and is associated with autism features and macro-orchidism after puberty.

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 Child and adolescent psychiatry: fragile X syndrome?

Learn each diagnosis by its core features and its first-line treatment. Then use the flashcards and the other Psychiatry 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.