Which genetic testing is used to detect Fragile X syndrome as part of the screening process for developmental delays?

Which genetic testing is used to detect Fragile X syndrome as part of the screening process for developmental delays?

• A. DNA studies to detect fragile X syndrome
• B. Chromosomal studies or karyotyping
• C. Urine amino acid levels
• D. Serum antinuclear antibody levels

Answer: (C)

Fragile X syndrome results from an expansion of CGG repeats in the FMR1 gene on the X chromosome. Because this change is in the DNA sequence itself, the best way to detect it is through DNA-based testing that directly measures the number of CGG repeats. PCR can identify smaller expansions, and Southern blot is used for larger expansions or to assess methylation status, which affects expression. Karyotyping looks for larger chromosome structure changes and would not reliably detect CGG repeat expansions. Urine amino acid testing screens for metabolic disorders, and serum ANA testing screens for autoimmune conditions—neither identifies Fragile X. In the context of developmental delays, confirming Fragile X with DNA testing provides a precise diagnosis, guiding management and genetic counseling.

Fragile X syndrome results from an expansion of CGG repeats in the FMR1 gene on the X chromosome. Because this change is in the DNA sequence itself, the best way to detect it is through DNA-based testing that directly measures the number of CGG repeats. PCR can identify smaller expansions, and Southern blot is used for larger expansions or to assess methylation status, which affects expression.

Karyotyping looks for larger chromosome structure changes and would not reliably detect CGG repeat expansions. Urine amino acid testing screens for metabolic disorders, and serum ANA testing screens for autoimmune conditions—neither identifies Fragile X.

In the context of developmental delays, confirming Fragile X with DNA testing provides a precise diagnosis, guiding management and genetic counseling.