Genomic imprinting: Prader-Willi syndrome: practice question

A Biochemistry 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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Biochemistry · Genomic imprinting: Prader-Willi syndrome

Sample item

A 2-year-old boy had profound neonatal hypotonia with poor suck and now shows hyperphagia, obesity, almond-shaped eyes and small genitalia. Prader-Willi syndrome is suspected. Which test is the best first confirmatory investigation?

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

The answer and the reasoning

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

Prader-Willi syndrome arises from loss of the paternally expressed genes at 15q11-q13: paternal deletion (about 70%), maternal uniparental disomy (about 25%) or an imprinting defect. Methylation testing detects all these mechanisms because only the maternal methylation pattern is present when the paternal contribution is missing, whereas FISH detects only deletions. Further testing then identifies the mechanism for recurrence counselling.

Option by option

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

    Methylation analysis identifies the abnormal pattern in every mechanism.

  • B. FISH for the 15q11-q13 region

    FISH misses uniparental disomy and imprinting defects.

  • C. Serum insulin-like growth factor and leptin levels

    Hormone levels are not diagnostic of this condition.

  • D. G-banded karyotype

    A karyotype misses most deletions and all uniparental disomy.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, DNA methylation analysis of the 15q11-q13 region. Prader-Willi syndrome arises from loss of the paternally expressed genes at 15q11-q13: paternal deletion (about 70%), maternal uniparental disomy (about 25%) or an imprinting defect.

Why is "FISH for the 15q11-q13 region" not the answer?

FISH misses uniparental disomy and imprinting defects.

Why is "Serum insulin-like growth factor and leptin levels" not the answer?

Hormone levels are not diagnostic of this condition.

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

A karyotype misses most deletions and all uniparental disomy.

What concept does this question test?

It tests Genomic imprinting: Prader-Willi syndrome in Biochemistry. Methylation testing detects all these mechanisms because only the maternal methylation pattern is present when the paternal contribution is missing, whereas FISH detects only deletions. Further testing then identifies the mechanism for recurrence counselling.

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 Genomic imprinting: Prader-Willi syndrome?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.