Classic ADHD brain differences

"Attention-Deficit/Hyperactivity Disorder is not a single pathophysiological entity and appears to have a complex etiology. There are multiple genetic and environmental risk factors with small individual effect that act in concert to create a spectrum of neurobiological liability. Structural imaging studies show that brains of children with Attention-Deficit/Hyperactivity Disorder are significantly smaller than unaffected controls. The prefrontal cortex, basal ganglia and cerebellum are differentially affected and evidence indicating reduced connectivity in white matter tracts in key brain areas is emerging. 

Genetic, pharmacological, imaging, and animal models highlight the important role of dopamine dysregulation in the neurobiology of Attention-Deficit/Hyperactivity Disorder. To date, stimulants are the most effective psychopharmacological treatments available for Attention-Deficit/Hyperactivity Disorder. Currently only immediate release methylphenidate and atomoxetine are approved for the treatment of ADHD in Italy. Drug treatment should always be part of a comprehensive plan that includes psychosocial, behavioural and educational advice and interventions."  source

 Rule Outs:

"Sometimes ADHD-like symptoms are exhibited by patients with established neurogenetic disorders such as Tuberous Sclerosis Complex, Neurofibromatosis I, Turner Syndrome, Williams Syndrome, Velocardiofacial syndrome, Prader-Willy syndrome, and Fragile X Syndrome." source

• Neurofibromatosis I, A disorder of the nervous system affecting development of nerve cell tissues.
• Turner Syndrome, A condition affecting female, due to partially or completely missing sex chromosome. Most of them can lead normal life with regular medical care.
• Williams Syndrome, A rare genetic disorder that causes developmental and learning disabilities.
• Velocardiofacial syndrome is the result of a microdeletion of DNA on a single copy of chromosome 22q11.2.
• Prader-Willy syndrome, A genetic disorder that affects many parts of the body and their growth. It causes mental and behavioral problems.
• Fragile X Syndrome, A genetic condition inherited from parents which results in various developmental problems like intellectual disabilities and cognitive impairment.

  Methylphenidate, sold under the brand name Ritalin and Concerta among others, is a stimulant drug used to treat attention deficit-hyperactivity disorder (ADHD) and narcolepsy. It is a first-line medication for ADHD. It may be taken by mouth or applied to the skin, and different formulations have varying durations of effect. It is also used recreationally as a performance-enhancer and/or as a euphoriant.

10.1186/1824-7288-36-79.pdf (springer.com)