The purpose of this study is to test an investigational medication in children and adolescents aged 5-17 years old with irritability associated with autism spectrum disorder (ASD). The name of the medication being tested in this study is pimavanserin. This medication is investigational because it has not been approved by the FDA for the treatment of irritability associated with autism. The main risks of this study are from the medication, which include nausea, dizziness, and sleepiness.