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: Biochemistry
Deficiency of which causes carcinoid syndrome 
Select and explain. 
A. Niacin, B. Thiamine, C. Riboflavin, D. Vit c
A: Niacin
Answer: A: Niacin 
Explanation: None