An 8-year-old boy has moderate intellectual disability, a long face with large prominent ears, and hyperextensible joints. He is anxious and avoids eye contact, with repetitive hand mannerisms. His maternal uncle also has intellectual disability. Which investigation will confirm the diagnosis?

• A Karyotype with high-resolution banding
• B FMR1 gene CGG repeat expansion testing ✓
• C Serum TSH and free T4
• D Urinary metabolic screening

Explanation

The long face, large ears, joint laxity, autistic-like social behavior, X-linked family history through the maternal line, and postpubertal macroorchidism point to fragile X syndrome, caused by CGG trinucleotide expansion in the FMR1 gene, confirmed by molecular testing. Standard karyotyping usually misses the fragile site, making option A inadequate. The maternal uncle being affected fits X-linked inheritance and argues against autosomal conditions like Down syndrome.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.