Is it Alzheimer's or another dementia form? Why doctors need to distinguish

Is it Alzheimer's or another dementia form? Why doctors need to distinguish:

Dementia Big
Alzheimer’s disease now affects an estimated 5.5 million Americans, and after decades of feverish work, researchers have so far failed to find a treatment that halts or reverses the inexorable loss of memory, function and thinking ability that characterize this feared illness.

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