**Question:** Genetic abnormality in fragile X syndrome-

A. Mutation in X chromosome
B. Hypothyroidism
C. Altered G protein signaling
D. Autoimmune disorder

**Core Concept:**
Fragile X syndrome (FXS) is a primary genetic disorder caused by a specific mutation in the FMR1 gene located on the X chromosome. The FMR1 gene encodes for the protein FMRP, which is crucial for proper synaptic development and function. In FXS, a full mutation occurs when over 200 CGG repeats are present in the FMR1 gene promoter region.

**Why the Correct Answer is Right:**
The correct answer, A - Mutation in X chromosome, highlights the genetic basis of the syndrome. FXS is caused by a full mutation in the FMR1 gene located on the X chromosome. This mutation leads to the absence or reduced expression of FMRP, which disrupts normal synaptic development and function, resulting in the characteristic features of FXS.

**Why Each Wrong Option is Incorrect:**

B. Hypothyroidism - FXS is primarily a genetic disorder, not a thyroid disorder. Hypothyroidism is unrelated to FMR1 gene mutations and FMRP deficiency.

C. Altered G protein signaling - Although FXS involves impaired G protein signaling, this option is too broad and does not specifically address the genetic cause of the syndrome. The core genetic abnormality lies in the FMR1 gene mutation and FMRP deficiency.

D. Autoimmune disorder - FXS is a primary genetic disorder, not an autoimmune condition. Autoimmune disorders are unrelated to FMR1 gene mutations and FMRP deficiency.

**Clinical Pearl:**
FMRP plays a crucial role in synaptic development, plasticity, and function. The genetic mutation in FXS leads to the deficiency of FMRP, causing cognitive, behavioral, and physical impairments seen in affected individuals.

**Correct Answer:**
A. Mutation in X chromosome

**Answer Explanation:**
The correct answer (option A) addresses the genetic basis of the syndrome, highlighting the mutation in the X chromosome and the FMR1 gene. FMR1 gene mutation leads to the deficiency of FMRP, which is responsible for the characteristic features of FXS, including cognitive, behavioral, and physical impairments.