A 6-year-old boy is evaluated for intellectual disability, speech delay, and social anxiety. Physical examination reveals macroorchidism, a long face with a prominent mandible, and large ears. Genetic testing confirms a trinucleotide repeat expansion. Which specific nucleotide repeat expansion causes this syndrome?

• A CTG
• B CGG ✓
• C CAG
• D GAA

Explanation

Fragile X syndrome is caused by an expansion of the CGG trinucleotide repeat in the FMR1 gene on the X chromosome. This expansion silences the gene, causing intellectual disability and the characteristic facial and testicular features. CTG repeats cause myotonic dystrophy. CAG repeats cause Huntington disease and spinocerebellar ataxias. GAA repeats cause Friedreich ataxia.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.