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
Korsakoff's psychosis is caused by deficiency of vitamin 
Select and explain. 
A. Vitamin B1, B. Vitamin B2, C. Vitamin C, D. Vitamin D
A: Vitamin B1
Answer: A: Vitamin B1 
Explanation: None