This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
Which of the following is a cause of reversible dementia? 
Select and explain. 
A. Subacute combined degeneration., B. Picks disease, C. Creutzfeldt - Jakob disease., D. Alzheimer's disease
A: Subacute combined degeneration.
Answer: A: Subacute combined degeneration. 
Explanation: Answer is A (Subacute combined degeneration) `Subacute combined degeneration' is caused by vitamin B12 deficiency. Vitamin deficiencies including vitamin B,2 deficiency constitute potentially reversible causes of dementia.