What condition occurs only in females and is characterized by normal development until 6 to 18 months of age, followed by a decline in mental abilities?

Rett syndrome is a neurodevelopmental disorder that predominantly affects females and is characterized by normal early growth and development for the first six to eighteen months of life. Following this period, individuals with Rett syndrome typically experience a significant regression in cognitive and motor skills. This regression often includes a loss of purposeful hand skills, social engagement, and speech abilities, while also presenting with repetitive hand movements, gait abnormalities, and seizures.

The unique aspect of Rett syndrome is its almost exclusive occurrence in females, attributed to its association with mutations in the MECP2 gene located on the X chromosome. Males who inherit similar mutations often do not survive to birth or exhibit severe forms of the condition that are incompatible with life, making Rett syndrome a condition that clearly differentiates itself from the other options listed, which can affect both genders and often do not follow the same pattern of development and regression.

Understanding these characteristics helps clarify why Rett syndrome is the only condition among the options presented that fits the specified criteria of being exclusive to females and exhibiting a specific timeline of normal development followed by a decline in mental abilities.