The medical disorder that makes a person be unable to focus is what is called ADHD. This disorder in full is attention deficit hyperactive disorder (ADHD).
A child's behavioral symptoms is the basis for an ADHD test. The medical professional who is administering the ADHD test will want to go through a comprehensive list of behavioral symptoms with you to see how man of these symptoms related to your child.
One of the common things that we mostly associate on ADHD would be hyperactivity. Though hyperactivity is usually present in many children with ADHD, being very active however does not really mean that you have a diagnosis.