The parts of the brain implicated in ADHD, such as the prefrontal cortex and the basal ganglia, feature large amounts of __dopamine__ activity. Many of the medications used to treat ADHD work by boosting the activity of this chemical.
Explanation:
Attention deficit hyperactivity disorder (ADHD) is a medical condition in which patient brain has differences in brain development and activity. This can affect ability of sitting still, attention and self control. Moreover, children can be affected at home, at school and in friendships. (See attached Detailed info graph)
The generalization that all organisms are composed of cells was established through multiple observations in various kinds of animals and plants. A controlled experiment of finding cells in all the kinds of living organisms is not possible or is infeasible as it will require prior knowledge of all the kinds of organisms inhabiting Earth and the effort to catch and bring them in the controlled environment of the lab for testing.