**Core Concept**
Rett syndrome is a rare genetic disorder that affects brain development, leading to severe mental and physical disability. It is caused by mutations in a specific gene that plays a crucial role in neuronal development and function.

**Why the Correct Answer is Right**
Rett syndrome is caused by mutations in the _MECP2_ gene, which is located on the X chromosome. The MECP2 protein is essential for the development and maintenance of neurons, and mutations in this gene disrupt the normal functioning of these cells. This leads to the characteristic symptoms of Rett syndrome, including loss of speech, motor skills, and cognitive impairment. The MECP2 gene is a transcription factor that binds to methylated DNA and regulates gene expression in neurons.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a gene associated with Rett syndrome. While there are other genes involved in neurodevelopmental disorders, MECP2 is the primary gene linked to Rett syndrome.
* **Option B:** This option is incorrect because it is not a known gene associated with Rett syndrome. Although it may be involved in other neurological conditions, it is not the primary gene responsible for Rett syndrome.
* **Option C:** This option is incorrect because it is not a gene associated with Rett syndrome. While it may be involved in other neurodegenerative disorders, it is not the primary gene linked to Rett syndrome.

**Clinical Pearl / High-Yield Fact**
Rett syndrome is a classic example of a genetic disorder caused by a mutation in a single gene, highlighting the importance of genetic testing in the diagnosis and management of this condition.

**Correct Answer: D. MECP2**