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 is characterized by social and communication deficits, but decelerated head growth in early infancy is not a typical feature of ASD. Choice B reason: Rett syndrome is a neurodevelopmental disorder affecting primarily girls, often presenting with normal early development followed by deceleration of head growth and regression of motor and communication skills, making this the correct diagnosis. Choice C reason: Global developmental delay is a broad diagnosis for children under 5 with significant delays across multiple developmental domains but does not specifically account for the distinctive regression and head growth deceleration seen in Rett syndrome. Choice D reason: Pervasive developmental disorder is an outdated term; while it historically included autism and Rett syndrome, current diagnostic criteria separate Rett syndrome as a distinct disorder.