Brain scans of people exposed to certain neurotoxins reveal brain damage identical to that found in people suffering from Parkinson' s disease. This fact shows not only that these neurotoxins cause this type of brain damage, but also that the brain damage itself causes Parkinson' s disease. Thus brain scans can be used to determine who is likely to develop Parkinson' s disease.
The argument contains the reasoning error of overestimating the importance of early diagnosis in determining appropriate treatments for people suffering from Parkinson's disease.