Genetics: genomic imprinting: 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: genomic imprinting

Sample item

A 2-month-old infant has marked hypotonia, a poor suck, almond-shaped eyes and hypogenitalism. Prader-Willi syndrome is suspected. What is the most appropriate first diagnostic test?

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

The answer and the reasoning

B. DNA methylation analysis of the 15q11-q13 region

Prader-Willi syndrome results from loss of the paternally expressed genes at 15q11-q13, through paternal deletion (about 70%), maternal uniparental disomy or an imprinting defect. Methylation analysis detects all these mechanisms, so it is the first test, and further studies define the subtype. Karyotype misses small deletions.

Option by option

  • A. G-banded karyotype

    A routine karyotype cannot detect the small deletion or uniparental disomy in most patients.

  • B. DNA methylation analysis of the 15q11-q13 region (correct)

    Correct. Abnormal methylation at 15q11-q13 confirms the diagnosis whatever the mechanism.

  • C. FISH for the 22q11.2 region

    FISH for 22q11.2 detects the deletion of DiGeorge syndrome.

  • D. PCR for CGG repeats in FMR1

    CGG repeat testing of FMR1 is for 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 B, DNA methylation analysis of the 15q11-q13 region. Prader-Willi syndrome results from loss of the paternally expressed genes at 15q11-q13, through paternal deletion (about 70%), maternal uniparental disomy or an imprinting defect.

Why is "G-banded karyotype" not the answer?

A routine karyotype cannot detect the small deletion or uniparental disomy in most patients.

Why is "FISH for the 22q11.2 region" not the answer?

FISH for 22q11.2 detects the deletion of DiGeorge syndrome.

Why is "PCR for CGG repeats in FMR1" not the answer?

CGG repeat testing of FMR1 is for fragile X syndrome.

What concept does this question test?

It tests Genetics: genomic imprinting in Anatomy. Methylation analysis detects all these mechanisms, so it is the first test, and further studies define the subtype. Karyotype misses small deletions.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: genomic imprinting?

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.