A 7-year-old boy is brought to the child psychiatry clinic by his mother with complaints of inattention, hyperactivity, and impulsivity for the past 2 years. He is unable to sit still in class, frequently interrupts his teacher, and loses his school materials. On assessment, he meets DSM-5 criteria for ADHD. Which is the most common neurobiological abnormality implicated in the pathophysiology of ADHD?