A 2-year-old boy is brought to the pediatric clinic by his mother for a routine check-up. The family lives in an old house built before 1950 that is undergoing renovations. The mother reports that her son is often putting toys and objects in his mouth. On developmental assessment, the child has speech delay and difficulty following instructions. A blood test reveals a blood lead level of 20 µg/dL. Which of the following systems is most likely to be affected in this child, resulting in long-term complications?
