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: Physiology
FIGLU excretion in the urine, is an index of the deficiency of: 
Select and explain. 
A. Thiamine, B. Niacin, C. Pyridoxine, D. Folate
D: Folate
Answer: D: Folate 
Explanation: Folate