CNS: Huntington disease: practice question

A Pathology 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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Pathology · CNS: Huntington disease

Sample item

A 42-year-old man has chorea, depression and cognitive decline. His father developed similar illness at 35 and his grandfather in his fifties. Which genetic abnormality explains the pattern of inheritance?

ACGG repeat expansion in the FMR1 gene on the X chromosomeBGAA repeat expansion in the frataxin gene on chromosome 9CCTG repeat expansion in the DMPK gene on chromosome 19DCAG repeat expansion in the huntingtin gene on chromosome 4

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

The answer and the reasoning

D. CAG repeat expansion in the huntingtin gene on chromosome 4

Huntington disease is autosomal dominant, caused by CAG repeat expansion in the huntingtin gene on chromosome 4. Longer repeats cause earlier onset, and expansion is more marked on paternal transmission, producing anticipation. Pathology shows marked atrophy of the caudate nucleus and putamen with loss of medium spiny neurons.

Option by option

A. CGG repeat expansion in the FMR1 gene on the X chromosome
CGG expansion of FMR1 causes fragile X syndrome with intellectual disability.
B. GAA repeat expansion in the frataxin gene on chromosome 9
GAA expansion causes Friedreich ataxia, inherited recessively.
C. CTG repeat expansion in the DMPK gene on chromosome 19
CTG expansion of DMPK causes myotonic dystrophy type 1 with myotonia and cataracts.
D. CAG repeat expansion in the huntingtin gene on chromosome 4 (correct)
Correct. A CAG expansion on chromosome 4 with anticipation explains the family history.

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 Pathology question

What is the answer to this Pathology question?

The best answer is D, CAG repeat expansion in the huntingtin gene on chromosome 4. Huntington disease is autosomal dominant, caused by CAG repeat expansion in the huntingtin gene on chromosome 4.

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

CGG expansion of FMR1 causes fragile X syndrome with intellectual disability.

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

GAA expansion causes Friedreich ataxia, inherited recessively.

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

CTG expansion of DMPK causes myotonic dystrophy type 1 with myotonia and cataracts.

What concept does this question test?

It tests CNS: Huntington disease in Pathology. Longer repeats cause earlier onset, and expansion is more marked on paternal transmission, producing anticipation. Pathology shows marked atrophy of the caudate nucleus and putamen with loss of medium spiny neurons.

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 CNS: Huntington disease?

Learn each disease as a bundle: cause, key histology, markers, and the clinical hallmark. Then use the flashcards and the other Pathology 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.