Which of the following best describes the characteristic repeat sequence associated with Fragile X syndrome?

Fragile X syndrome is associated with a specific type of genetic mutation characterized by the expansion of a trinucleotide repeat sequence. In this case, the defining sequence is the repetition of 'CGG' in the FMR1 gene located on the X chromosome. Individuals with the condition typically have a large number of CGG repeats—more than 200, which leads to the silencing of the gene and results in the symptoms of Fragile X syndrome, including cognitive impairment and developmental delays.

The correct answer reflects the nature of the mutation that leads to the syndrome, emphasizing the crucial role of the CGG repeat expansion in its pathology. Understanding this repeat sequence is essential for genetic testing and counseling related to Fragile X syndrome, as it aids in the diagnosis and provides insights regarding inheritance patterns.