A 6-year-old girl is brought to the clinic by her parents, who report that she has not spoken at school since entering kindergarten 14 months ago. Her teacher confirms that the child does not speak to teachers or peers in the classroom, on the playground, or in the cafeteria, despite appearing to understand instructions and participating nonverbally. At home, the child speaks fluently and age-appropriately with family members and a few close family friends. Developmental milestones were met on time, and a recent speech-language evaluation found no abnormalities. Her parents describe her as shy and report that she clings to her mother in unfamiliar social situations. Which assessment finding would most strongly support a diagnosis of selective mutism?