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The problem with classifying ADD into this type of situation would imply that every classic symptom is the dominant, in indeed only symptom displayed by another problem. Say you have four outward symptoms of ADD, but they are just symptoms of other problems. It would mean that in every case the only symptom showing happens to be the same symptom of ADD. That would be rather far fetched. Now if there are other symptoms, then they are likely being ignored and thus ADD becomes a complex comorbidity that is very hard to unravel. I'm sure this happens, but as a rule, I really doubt that ADD is NOT its own unique problem. The evidence you can see is usually much to the opposite of the comorbidity theory.