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Fragile X syndrome — RACP Paediatrics MCQ

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ModerateGenetics and metabolicFragile X syndromeRACP Paediatrics

A 9-year-old boy has intellectual disability, anxiety, prominent ears and a long face. His mother reports two male relatives with learning difficulties; examination shows macro-orchidism. What is the most likely cause?

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Correct answer: E — CGG repeat expansion in the FMR1 gene

The phenotype and X-linked family pattern suggest fragile X syndrome due to CGG repeat expansion in FMR1. Williams syndrome from 7q11.23 deletion causes a different social and cardiovascular phenotype. The pearl is that identifying fragile X has implications for maternal carrier status and premature ovarian insufficiency risk in female relatives.

Reference: RCH Clinical Practice Guidelines; GeneReviews: Fragile X syndrome