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

A. CGG
B. GGC
C. CAG
D. GAA

**Correct Answer:** A. CGG

**Core Concept:**
Fragile X syndrome is a genetic disorder caused by an expansion of a trinucleotide repeat sequence, specifically the CGG repeat, in the FMR1 gene located on the X chromosome. The normal range for CGG repeats is between 5 and 50. When the number of CGG repeats increases, it leads to a higher risk of developing Fragile X syndrome.

**Why the Correct Answer is Right:**
The correct answer, CGG, refers to the trinucleotide repeat found in the FMR1 gene. This gene is responsible for encoding the protein FMRP, which plays a crucial role in synaptic plasticity and development. In individuals with Fragile X syndrome, the CGG repeats expand, leading to the silencing of the FMR1 gene and its associated protein. This results in the development of the characteristic features of the disorder, including intellectual disability, behavioral problems, and distinctive facial features.

**Why Each Wrong Option is Incorrect:**

A. CGG (Option B) is incorrect because it is the correct answer and not associated with Fragile X syndrome.
B. GGC (Option B) is incorrect because it is not the trinucleotide repeat found in the FMR1 gene.
C. CAG (Option C) is incorrect because it is the trinucleotide repeat associated with Huntington's disease and not Fragile X syndrome.
D. GAA (Option D) is incorrect because it is the trinucleotide repeat associated with Friedreich's ataxia, a different genetic disorder.

**Clinical Pearl:**
Understanding the role of trinucleotide repeats in genetic disorders is crucial for diagnosing and treating these conditions. Trinucleotide repeats can expand in size due to errors in DNA replication, leading to the development of various genetic disorders like Fragile X syndrome, Huntington's disease, and Friedreich's ataxia.

**Why the Correct Answer is Right:**
CGG is the correct answer because it represents the trinucleotide repeat found in the FMR1 gene, which leads to the development of Fragile X syndrome when the repeats expand. This expansion results in the silencing of the FMR1 gene and its associated protein, FMRP, leading to the characteristic features of Fragile X syndrome.

**Why Each Wrong Option is Incorrect:**
Option B (GGC) is incorrect as it is not the trinucleotide repeat found in the FMR1 gene. Option C (CAG) is incorrect because it represents the trinucleotide repeat associated with Huntington's disease, a different genetic disorder. Option D (GAA) is incorrect as it is the trinucleotide repeat found in the GAA gene, which is associated with Friedreich's ataxia, another genetic disorder.