**Core Concept**
Rett's syndrome is a rare genetic disorder that affects brain development, leading to severe physical and mental disability. It is characterized by a unique pattern of brain function, behavior, and physical symptoms. The underlying cause is a mutation in the MECP2 gene on the X chromosome.

**Why the Correct Answer is Right**
The MECP2 gene provides instructions for making a protein called methyl-CpG-binding protein 2 (MeCP2), which is essential for brain development. In Rett's syndrome, the mutation leads to a loss of function of MeCP2, disrupting normal brain development and function. This results in a range of symptoms, including intellectual disability, loss of speech, and characteristic hand movements. The brain's inability to develop and function properly leads to the distinctive features of Rett's syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because Rett's syndrome is not primarily caused by a lack of oxygen to the brain (hypoxia). While hypoxia can cause brain damage, it is not the underlying cause of Rett's syndrome.
**Option B:** This is incorrect because Rett's syndrome is not a result of a viral infection. While some genetic disorders are caused by viral infections, Rett's syndrome is a genetic disorder caused by a mutation in the MECP2 gene.
**Option C:** This is incorrect because Rett's syndrome is not a form of autism spectrum disorder (ASD). While both conditions involve developmental delays and intellectual disability, they are distinct disorders with different underlying causes and symptoms.

**Clinical Pearl / High-Yield Fact**
Rett's syndrome is a classic example of a genetic disorder caused by a mutation in a specific gene. It highlights the importance of genetic testing in diagnosing and managing rare genetic disorders.

**Correct Answer: D. Rett's syndrome is a rare genetic disorder caused by a mutation in the MECP2 gene on the X chromosome.**