A 3-year-old boy has moderate intellectual disability, macrocephaly, prominent ears, a long face, and macroorchidism on examination. Which genetic abnormality is most likely responsible?

• A CGG trinucleotide repeat expansion on Xq27.3 ✓
• B Trisomy 18
• C Trisomy 21
• D Microdeletion on chromosome 22q11.2

Explanation

Fragile X syndrome, caused by a CGG trinucleotide repeat expansion in the FMR1 gene at Xq27.3, is the most common inherited cause of intellectual disability. The characteristic physical features include a long face, prominent ears, macrocephaly, and postpubertal macroorchidism. Trisomy 21 causes Down syndrome with different facial features. 22q11.2 deletion causes DiGeorge syndrome.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.