**Question:** Trinucleotide repeat seen in Fragile X syndrome is

A. CGG
B. CCG
C. CGA
D. CCT

**Core Concept:**
Fragile X syndrome is a genetic disorder characterized by intellectual disability, behavioral problems, and physical features. The pathogenesis of the disease is linked to the التوسعة repetitive sequence trinucleotide repeats, specifically CGG, in the FMR1 gene located on the X-chromosome.

**Why the Correct Answer is Right:**
The correct answer, A. CGG, represents the trinucleotide repeat found in the FMR1 gene in Fragile X syndrome. The CGG repeat expansion leads to the silencing of the FMR1 gene and the absence of its protein product, fragile X mental retardation protein (FMRP). This results in the pathophysiology of the syndrome, including cognitive impairment, behavior abnormalities, and other features.

**Why Each Wrong Option is Incorrect:**
Option B, CCG, is incorrect because it is another trinucleotide repeat sequence found in the FMR1 gene but does not cause Fragile X syndrome. The pathophysiology is specific to the CGG repeat expansion.

Option C, CGA, is incorrect because it is another trinucleotide repeat sequence found in the FMR1 gene but does not cause Fragile X syndrome. The pathophysiology is specific to the CGG repeat expansion.

Option D, CCT, is incorrect as it is another trinucleotide repeat sequence found in the FMR1 gene but does not cause Fragile X syndrome. The pathophysiology is specific to the CGG repeat expansion.

**Clinical Pearl:**

Understanding the specific trinucleotide repeat sequence (CGG) associated with Fragile X syndrome is crucial for differential diagnosis and genetic counseling. The التوسعة repeat size also determines the severity of the syndrome - larger repeats are associated with more severe manifestations.

**Correct Answer:** A. CGG

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For further reference, here is a brief overview of the different trinucleotide repeat sequences found in the FMR1 gene:

1. Normal alleles (non-affected individuals): Trinucleotide repeats are typically between 5-54 CGG repeats.
2. Premutation alleles: CGG repeats between 55-200. Premutation carriers have an increased risk of developing Fragile X-associated primary ovarian insufficiency (FXPOI) or Fragile X-associated tremor/ataxia syndrome (FXTAS).
3. Full mutation alleles: CGG repeats greater than 200, which is associated with Fragile X syndrome and its related disorders.

In summary, the correct answer is CGG as it represents the trinucleotide repeat that causes Fragile X syndrome. Understanding the different trinucleotide repeat ranges is crucial for accurate diagnosis and risk assessment in genetic counseling.