Final answer:
Grave's disease is not a common cause of dementia, unlike diabetes mellitus, Parkinson's disease, and stroke, which all can lead to cognitive decline.
Step-by-step explanation:
All of the following are common causes of dementia EXCEPT Grave's disease. While diabetes mellitus can lead to cognitive decline due to vascular changes, Parkinson's disease is recognized for its neurodegenerative impact that often leads to dementia, and stroke can cause vascular dementia due to interrupted blood supply to the brain, Grave's disease does not commonly lead to dementia. Parkinson's disease specifically is characterized by the loss of dopamine neurons in the substantia nigra, which can cause not only motor symptoms but also cognitive issues like dementia. Robert undergoing a test to determine if he is diabetic is relevant as diabetes is a risk factor for dementia, but it does not directly cause it as Grave's disease does not.