Pigmentary disorders: McCune-Albright syndrome: practice question

A Dermatology 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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Dermatology · Pigmentary disorders: McCune-Albright syndrome

Sample item

A 7-year-old girl has vaginal bleeding with breast development, a large café-au-lait macule with a jagged border that stops at the midline, and a polyostotic bone lesion on radiograph showing a ground-glass appearance. Which genetic defect underlies this condition?

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

The answer and the reasoning

C. Somatic activating mutation of GNAS

McCune-Albright syndrome results from a post-zygotic activating mutation in GNAS, encoding the Gs alpha subunit. Mosaicism produces segmental café-au-lait macules with irregular borders, polyostotic fibrous dysplasia and endocrine overactivity such as precocious puberty. NF1 spots, in contrast, have smooth borders.

Option by option

  • A. Loss-of-function mutation of STK11

    STK11 mutation causes Peutz-Jeghers syndrome with mucocutaneous freckling and polyps.

  • B. Mutation of TSC2

    TSC2 mutation causes tuberous sclerosis with ash-leaf macules and angiofibromas.

  • C. Somatic activating mutation of GNAS (correct)

    Correct. A mosaic GNAS activating mutation drives the skin, bone and endocrine features.

  • D. Loss of neurofibromin (NF1)

    NF1 loss gives smooth-edged café-au-lait macules, axillary freckling and neurofibromas.

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 Dermatology question

What is the answer to this Dermatology question?

The best answer is C, Somatic activating mutation of GNAS. McCune-Albright syndrome results from a post-zygotic activating mutation in GNAS, encoding the Gs alpha subunit.

Why is "Loss-of-function mutation of STK11" not the answer?

STK11 mutation causes Peutz-Jeghers syndrome with mucocutaneous freckling and polyps.

Why is "Mutation of TSC2" not the answer?

TSC2 mutation causes tuberous sclerosis with ash-leaf macules and angiofibromas.

Why is "Loss of neurofibromin (NF1)" not the answer?

NF1 loss gives smooth-edged café-au-lait macules, axillary freckling and neurofibromas.

What concept does this question test?

It tests Pigmentary disorders: McCune-Albright syndrome in Dermatology. Mosaicism produces segmental café-au-lait macules with irregular borders, polyostotic fibrous dysplasia and endocrine overactivity such as precocious puberty. NF1 spots, in contrast, have smooth borders.

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 Pigmentary disorders: McCune-Albright syndrome?

Learn each condition by its primary lesion, site and signature sign. Then use the flashcards and the other Dermatology 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.