A 7-year-old boy is brought to the child psychiatry clinic by his mother with complaints of inattention, difficulty completing schoolwork, and frequent fidgeting in class for the past 2 years. The teacher reports that he is unable to sit still during lessons and often interrupts other children. On evaluation, he meets DSM-5 criteria for Attention-Deficit/Hyperactivity Disorder. Which is the most common neurobiological abnormality found in children with ADHD?