A 5-month-old girl presents with decreased head growth, truncal discoordination, and decreased social interaction. What is the likely diagnosis?

The diagnosis of Rett's disorder is supported by the key clinical features presented in this case. This neurodevelopmental condition primarily affects females and is characterized by normal early development followed by a period of regression. The decreased head growth observed in the child aligns with the typical pattern seen in Rett's disorder, where microcephaly can develop as the disorder progresses. Truncal discoordination, which refers to difficulties with gross motor skills and balance, is also a hallmark of Rett's disorder as motor function becomes impaired after the initial period of normality. Furthermore, the decreased social interaction is a significant indicator, as children with Rett's often lose social engagement and communication skills that they initially acquired.

This combination of symptoms in a 5-month-old girl strongly suggests Rett's disorder, which would not typically present in the same manner for the other options. Autism spectrum disorder generally presents with social communication deficits but does not typically feature the regression of acquired skills and increased motor dysfunction seen in Rett's. Cerebral palsy usually involves motor deficits related to the brain's development pre- or perinatally and doesn’t typically present with the same regression in social interaction as seen in Rett's. Down syndrome presents with distinct phenotypic features and cognitive delays but would not typically