Phenylketonuria is a rare inborn error of metabolism. The build-up of phenylalanine in the blood and body tissues can have significant impact on the brain's development. High phenylalanine levels have been shown to be associated with an increase in neuropsychiatric symptoms, including mood, anxiety, and attention problems; decreased social competence; and low self-esteem. This case report highlights such a presentation in an adolescent with phenylketonuria. Copyright © 2019 National Association of Pediatric Nurse Practitioners. Published by Elsevier Inc. All rights reserved.