One of the following is a reversible cause of dementia -
Reversible causes of dementia

Surgically treatable:- Normal pressure hydrocephalus, brain tumors (frontal lobe tumor), meningioma, subdural hematoma due to head injury, hydrocephalus.
Medically treatable:- Depression, hypothyroidism, alcohol abuse, vitamin B 12/folate/Niacin deficiency, any metabolic or endocrine disturbance, neurosyphilis, Hashimoto's encephalopathy, Wilson's disease, celiac disease, Whipple's disease, chronic meningoencephalitis, drugs and toxin (toxic dementia).

Irreversible causes of dementia:- Alzheimer's disease, vascular (multi-infarct) dementia, Parkinsonism, Huntington's chorea, Lewy body dementia, Pick's disease.
Toxic dementia
Alzheimer's ds.
Multi-infarct dementia
Pick ds.