Genetics: trinucleotide repeat disorders: practice question

A Anatomy 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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Anatomy · Genetics: trinucleotide repeat disorders

Sample item

A 7-year-old boy has intellectual disability, autistic features, a long narrow face, prominent ears and hyperextensible finger joints. His maternal uncle is similarly affected. What is the molecular basis?

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

The answer and the reasoning

D. CGG repeat expansion above 200 in FMR1 with promoter hypermethylation

Fragile X syndrome is an X-linked disorder caused by expansion of a CGG repeat in the 5' untranslated region of FMR1. More than 200 repeats (full mutation) lead to hypermethylation and silencing, with loss of FMRP. Premutation carriers (55 to 200 repeats) can have tremor-ataxia syndrome or premature ovarian insufficiency.

Option by option

  • A. CAG repeat expansion in HTT producing a polyglutamine tract

    CAG expansion in HTT causes Huntington disease, with adult-onset chorea and dementia.

  • B. GAA repeat expansion in intron 1 of FXN

    GAA expansion in FXN causes Friedreich ataxia, an autosomal recessive disease.

  • C. CTG repeat expansion in the 3' untranslated region of DMPK

    CTG expansion in DMPK causes myotonic dystrophy type 1.

  • D. CGG repeat expansion above 200 in FMR1 with promoter hypermethylation (correct)

    Correct. A full CGG expansion silences FMR1 and causes fragile X 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 Anatomy question

What is the answer to this Anatomy question?

The best answer is D, CGG repeat expansion above 200 in FMR1 with promoter hypermethylation. Fragile X syndrome is an X-linked disorder caused by expansion of a CGG repeat in the 5' untranslated region of FMR1.

Why is "CAG repeat expansion in HTT producing a polyglutamine tract" not the answer?

CAG expansion in HTT causes Huntington disease, with adult-onset chorea and dementia.

Why is "GAA repeat expansion in intron 1 of FXN" not the answer?

GAA expansion in FXN causes Friedreich ataxia, an autosomal recessive disease.

Why is "CTG repeat expansion in the 3' untranslated region of DMPK" not the answer?

CTG expansion in DMPK causes myotonic dystrophy type 1.

What concept does this question test?

It tests Genetics: trinucleotide repeat disorders in Anatomy. More than 200 repeats (full mutation) lead to hypermethylation and silencing, with loss of FMRP. Premutation carriers (55 to 200 repeats) can have tremor-ataxia syndrome or premature ovarian insufficiency.

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: trinucleotide repeat disorders?

Learn every nerve by its route, its muscles and what a lesion looks like at the bedside. That one habit answers a large share of questions. Then use the flashcards and the other Anatomy 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.