**Core Concept**
Rett syndrome is a rare genetic disorder affecting brain development, primarily affecting girls. It is characterized by intellectual disability, loss of purposeful hand use, and distinctive hand movements. The disorder is caused by mutations in a specific gene involved in brain development.

**Why the Correct Answer is Right**
Rett syndrome is caused by mutations in the MECP2 gene, which encodes a protein essential for brain development and function. The MECP2 gene is located on the X chromosome and is responsible for encoding the methyl-CpG-binding protein 2 (MeCP2), a transcriptional regulator that plays a critical role in neuronal development and maintenance. Mutations in the MECP2 gene disrupt normal brain development, leading to the characteristic symptoms of Rett syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the MECP2 gene is not associated with other conditions such as autism or fragile X syndrome.
**Option B:** This option is incorrect because the MECP2 gene is not involved in the pathogenesis of other neurodegenerative disorders such as Alzheimer's disease or Parkinson's disease.
**Option C:** This option is incorrect because the MECP2 gene is not located on chromosome 15, but rather on the X chromosome.
**Option D:** This option is incorrect because the MECP2 gene is not associated with other genetic disorders such as Down syndrome or Turner syndrome.

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

**Correct Answer: C. MECP2**