**Core Concept**
The CGG repeat sequence is a trinucleotide repeat expansion associated with a neurodegenerative disorder. This condition is characterized by an expansion of a CGG repeat in the 5' untranslated region of the FMR1 gene, leading to methylation and silencing of the gene. The resulting deficiency of the FMR1 protein is responsible for the neurodevelopmental and behavioral manifestations of the disorder.

**Why the Correct Answer is Right**
The correct answer is associated with Fragile X syndrome, a leading cause of inherited intellectual disability. The CGG repeat expansion in the FMR1 gene leads to hypermethylation of the gene, resulting in silencing of the gene and a deficiency of the FMR1 protein. This protein is essential for normal brain development and function, and its deficiency leads to the characteristic features of Fragile X syndrome, including intellectual disability, behavioral problems, and physical characteristics such as a long face and large ears.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the CGG repeat sequence is not associated with Huntington's disease, which is caused by an expansion of a CAG repeat in the Huntingtin gene.
**Option B:** This option is incorrect because the CGG repeat sequence is not associated with Friedreich's ataxia, which is caused by an expansion of a GAA repeat in the FXN gene.
**Option C:** This option is incorrect because the CGG repeat sequence is not associated with myotonic dystrophy, which is caused by an expansion of a CTG repeat in the DMPK gene.

**Clinical Pearl / High-Yield Fact**
A key feature of Fragile X syndrome is the presence of a CGG repeat expansion in the FMR1 gene. This expansion is typically above 200 repeats, and individuals with 55-200 repeats are considered carriers. The risk of expansion increases with the number of repeats, and individuals with a family history of Fragile X syndrome should be screened for the CGG repeat expansion.

**Correct Answer:** D. Fragile X syndrome.