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
[{'content': 'What is the most common cause of increased red cell protoporphyrin level ? \nSelect and explain. \nA. Iron deficiency, B. Thiamine deficiency, C. Reactive lymphocytosis, D. Drug toxicity', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The most common causes of increased red cell protoporphyrin levels are absolute or relative iron deficiency and lead poisoning. Reference: Harrisons Principles of Internal Medicine, 18th Edition, Page 847', 'role': 'assistant'}]