Alzheimer's is most common cause of dementia but..

Tour de force analysis of dementia case link

This NEJM Case study is extremely informative and instructive about the differential diagnosis of dementia. 

In my Geriatrics private practice, I concentrated first on checking for the reversible causes of dementia. 

The NEJM rules all of these out. 

Before brain scanning and histology, Dr. Brice offers a diagnosis:

Next step Brain Scan

 In my practice, I usually ordered a CAT scan and an MRI.

Of course, I relied on the radiologist's interpretation.

I have willed my brain for autopsy as part of the non-treatment I have participated in. Below is not my autopsy report. 

The diagnosis with Alzeheimer's is ultimately made with slides of the brain.  Below a different diagnosis is found. 

Below is from the internet, not the Case Study.