A 4-year-old boy has global developmental delay, macrocephaly, long face, large everted ears, and joint laxity. Family history shows an affected maternal uncle. DNA analysis would most likely show which abnormality?

• A Expansion of CAG trinucleotide repeats in the HTT gene
• B Expansion of CGG trinucleotide repeats in the FMR1 gene ✓
• C Deletion of 15q11-q13 on the paternal chromosome
• D Point mutation in the MECP2 gene

Explanation

Fragile X syndrome, the commonest inherited cause of intellectual disability in boys, is due to CGG trinucleotide expansion (>200 full mutation) with methylation silencing of the FMR1 gene at Xq27.3. The X-linked inheritance with affected maternal relatives fits fragile X. CAG expansion causes Huntington disease, MECP2 mutation causes Rett syndrome in girls, and paternal 15q11-q13 deletion causes Prader-Willi syndrome.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.