Neurology: Huntington disease genetics: practice question

A General Medicine 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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General Medicine · Neurology: Huntington disease genetics

Sample item

A 42-year-old man has progressive chorea, irritability, depression and cognitive decline. His father died in an institution after a similar illness, and MRI shows atrophy of the caudate heads. What is the underlying genetic abnormality?

ACAG trinucleotide repeat expansion in the HTT gene on chromosome 4BGAA repeat expansion in the frataxin gene on chromosome 9CCTG repeat expansion in the DMPK gene on chromosome 19DCGG 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

A. CAG trinucleotide repeat expansion in the HTT gene on chromosome 4

Huntington disease is autosomal dominant, caused by an expanded CAG repeat (36 or more) in the huntingtin gene on chromosome 4, producing a polyglutamine tract. It shows anticipation, especially with paternal transmission, and striatal neuronal loss. Chorea can be treated with tetrabenazine, and predictive testing requires counselling.

Option by option

A. CAG trinucleotide repeat expansion in the HTT gene on chromosome 4 (correct)
Correct. A CAG expansion in HTT on chromosome 4 causes Huntington disease.
B. GAA repeat expansion in the frataxin gene on chromosome 9
A GAA expansion in frataxin causes Friedreich ataxia, an autosomal recessive disorder.
C. CTG repeat expansion in the DMPK gene on chromosome 19
A CTG expansion in DMPK causes myotonic dystrophy type 1.
D. CGG repeat expansion in the FMR1 gene on the X chromosome
A CGG expansion in FMR1 causes fragile X syndrome and fragile X tremor ataxia 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.

Questions about this General Medicine question

What is the answer to this General Medicine question?

The best answer is A, CAG trinucleotide repeat expansion in the HTT gene on chromosome 4. Huntington disease is autosomal dominant, caused by an expanded CAG repeat (36 or more) in the huntingtin gene on chromosome 4, producing a polyglutamine tract.

Why is "GAA repeat expansion in the frataxin gene on chromosome 9" not the answer?

A GAA expansion in frataxin causes Friedreich ataxia, an autosomal recessive disorder.

Why is "CTG repeat expansion in the DMPK gene on chromosome 19" not the answer?

A CTG expansion in DMPK causes myotonic dystrophy type 1.

Why is "CGG repeat expansion in the FMR1 gene on the X chromosome" not the answer?

A CGG expansion in FMR1 causes fragile X syndrome and fragile X tremor ataxia syndrome.

What concept does this question test?

It tests Neurology: Huntington disease genetics in General Medicine. It shows anticipation, especially with paternal transmission, and striatal neuronal loss. Chorea can be treated with tetrabenazine, and predictive testing requires counselling.

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 Neurology: Huntington disease genetics?

For each disease know the classic presentation, the best test, and the first-line treatment. Then use the flashcards and the other General Medicine 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.