A mother brings in her 8-year-old son for a psychiatric foll…

A mother brings in her 8-year-old son for a psychiatric follow-up visit with the PMHNP. This is the fourth visit the PMHNP has had with the client, his mother, and his younger sister, now 7-8 months old. You notice that the sister has a decrease in head growth, along with stereotypic motions of the hands, often licking and slapping. She has also lost her language skills. What medical condition do you suspect she has?