<prompt> All are reversible causes of dementia exceptHypothyroidismHydrocephalusMeningoencephalitisAlzheimers disease  <response>: Alzheimer;s is the commonest cause of dementia The diagnosis of Alzheimer;s dementia is by an exclusion of all other causes of dementia as there is no distinct diagnostic clinical features or laboratory investigations. At present Alzheimer;s dementia is not considered a treatable disorder Ref: a sho textbook of psychiatry, Niraj Ahuja, 7th edition, pg no.23 