A 6-year-old boy has moderate intellectual disability, a long face, large everted ears, a prominent jaw, and hyperactivity. His maternal uncle also had intellectual disability. Which investigation is most appropriate to confirm the diagnosis?

• A Standard karyotype
• B Serum creatine kinase estimation
• C FMR1 gene CGG trinucleotide repeat analysis ✓
• D MECP2 gene sequencing

Explanation

The facial appearance, X-linked family history through the maternal line, and associated hyperactivity point to fragile X syndrome, caused by expansion of CGG repeats in the FMR1 gene. Standard karyotyping may show the fragile site but misses many cases and cannot quantify the expansion. MECP2 sequencing tests for Rett syndrome, which affects girls almost exclusively. Serum creatine kinase is relevant to Duchenne dystrophy, where weakness rather than cognitive profile dominates.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.