**Core Concept**
Fragile X syndrome is a genetic disorder caused by an expansion of a CGG repeat in the FMR1 gene on the X chromosome. This repeat expansion leads to methylation and silencing of the gene, resulting in the absence of the fragile X mental retardation protein (FMRP). The absence of FMRP is responsible for the cognitive and behavioral features associated with Fragile X syndrome.

**Why the Correct Answer is Right**
The correct answer is CGG because it is the specific triplet repeat expansion found in the FMR1 gene that causes Fragile X syndrome. The CGG repeat expansion leads to hypermethylation of the CpG islands in the promoter region of the FMR1 gene, resulting in gene silencing. This silencing is responsible for the absence of FMRP, which is essential for normal neuronal function and development. The absence of FMRP disrupts synaptic plasticity and leads to the cognitive and behavioral features associated with Fragile X syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** CTG is the triplet repeat expansion found in the DMPK gene, which causes myotonic dystrophy, a different genetic disorder. It is not associated with Fragile X syndrome.
**Option C:** CAG is the triplet repeat expansion found in genes such as huntingtin and androgen receptor, which cause Huntington's disease and spinal and bulbar muscular atrophy, respectively. It is not associated with Fragile X syndrome.
**Option D:** GAA is the triplet repeat expansion found in the FXN gene, which causes Friedreich's ataxia, a different genetic disorder. It is not associated with Fragile X syndrome.

**Clinical Pearl / High-Yield Fact**
It is essential to note that Fragile X syndrome is the most common cause of inherited intellectual disability and the most common known cause of autism. It is also associated with other conditions such as attention deficit hyperactivity disorder (ADHD), anxiety, and sleep disorders.

**Correct Answer:**
✓ Correct Answer: B. CGG