A 3-year-old girl has a presentation that meets criteria for autism spectrum disorder. However, a careful history identifies that her head growth began to decelerate around 8 months. What diagnosis will she receive?

Choice A reason: Autism spectrum disorder involves social communication deficits and restricted/repetitive behaviors, but does not typically include deceleration of head growth after normal early development. Choice B reason: This is correct. Rett syndrome, primarily affecting females, is characterized by normal early development followed by regression, deceleration of head growth (microcephaly), loss of purposeful hand skills, and social and motor impairments. The head growth deceleration at 8 months supports Rett syndrome over ASD. Choice C reason: Global developmental delay is a diagnosis for children under 5 with delays in multiple developmental domains but does not explain the specific pattern of regression and head growth deceleration seen here. Choice D reason: Pervasive developmental disorder is an outdated term encompassing ASD and similar conditions; it would not capture the distinctive features of Rett syndrome in this scenario.