Which of the following is NOT a form of autism spectrum disorder?

Down Syndrome is indeed not a form of autism spectrum disorder (ASD). It is a genetic condition caused by the presence of an extra chromosome 21, which leads to developmental delays and physical features distinct to the syndrome.

In contrast, Rett Syndrome, Pervasive Developmental Disorder not otherwise specified, and Asperger Syndrome all fall under the umbrella of autism spectrum disorders. Each of these conditions is characterized by challenges in social communication and restricted or repetitive patterns of behavior, which are key features of ASD. Recognizing Down Syndrome as a separate condition helps clarify the distinctions between developmental disorders, which is important for understanding individual needs and tailoring education and support approaches appropriately.