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
[{'content': 'Deficiency of which causes carcinoid syndrome \nSelect and explain. \nA. Niacin, B. Thiamine, C. Riboflavin, D. Vit c', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]