<prompt> In which of the following pervasive developmental disorder only one modality is impaired, others are normal -AutismAsperger syndromeRett syndromeAll of the above  <response>: Ans. is b i.e., Asperger syndromeo Pervasive development disorders include several clinically similar conditions which are characterized by three symptom clustors:-i Impairment of social interaction:- e.g., Lack of social smile or eye contact.ii Impairment of communication language and non-verbal:- Language problems, delayed or absent speech.iii Restricted repititive and styerotyped behavior, interests and activities:- e.g., Sterotypy.iv Another associated feature is mental retardation.o In Autism and Rett syndrome all three features are found. Where as:o In asperger syndrome -a Only Social ineractiion is impaired, where asb Intelligence is normal The only pervasive development disorder in which intelligence is normal.c No language impairments. 