All are reversible causes of dementia except

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)