Complex sentence: Worried by Lizzie's recent strange behavior, Polly brings her daughter to a psychologist who prescribes medication to rid her of Fred, whom the doctor and Polly believe is a figment of Lizzie's imagination.
Split into simpler sentences: Worried by Lizzie's recent strange behavior, Polly brings her daughter to a psychologist. The doctor prescribes medication to rid her of Fred, whom he and Polly believe is a figment of Lizzie's imagination.