## Core Concept
Rett syndrome is a genetic disorder that primarily affects girls, causing severe mental and physical disability. It is characterized by normal early growth and development followed by a slowing of development, intellectual disability, and physical disability. The disorder is caused by mutations in the MECP2 gene.

## Why the Correct Answer is Right
The MECP2 gene provides instructions for making a protein called methyl-CpG-binding protein 2, which is essential for normal brain function. Mutations in this gene lead to Rett syndrome by disrupting the normal development and maturation of nerve cells in the brain. This results in the characteristic features of the disorder, including intellectual disability, loss of speech, and repetitive hand movements.

## Why Each Wrong Option is Incorrect
- **Option A:** While Rett syndrome does involve intellectual disability and physical disability, stating it without specifying the genetic basis or the MECP2 gene mutation would be incomplete and not directly addressing the question.
- **Option B:** This option might discuss other aspects of neurodevelopmental disorders but does not specifically relate to the unique genetic cause or clinical features of Rett syndrome.
- **Option C:** Similar to Option B, it might touch on aspects of neurological disorders but lacks specificity regarding Rett syndrome's genetic underpinnings or clinical manifestations.

## Clinical Pearl / High-Yield Fact
A key clinical pearl for Rett syndrome is that it predominantly affects girls. This is because the MECP2 gene is located on the X chromosome, and girls have two X chromosomes. A mutation in one of the X chromosomes can lead to the disorder, while boys, having only one X chromosome, usually do not survive if they inherit the mutated gene.

**Correct Answer:** D. Rett syndrome is caused by mutations in the MECP2 gene.