A 7-year-old boy is brought to the child psychiatry clinic by his mother with complaints of inattention and hyperactivity for the past 18 months. The teacher reports that he frequently interrupts classmates, cannot sit still during lessons, and loses his pencils and homework regularly. At home, he fidgets constantly, talks excessively, and struggles to follow multi-step instructions. His mother denies any seizures, head injury, or developmental delay. Physical examination is unremarkable. Intelligence testing shows average cognitive ability. Which of the following is the MOST appropriate first-line pharmacological intervention for this child?