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
What is the most common cause of increased red cell protoporphyrin level ? 
Select and explain. 
A. Iron deficiency, B. Thiamine deficiency, C. Reactive lymphocytosis, D. Drug toxicity
A: Iron deficiency
Answer: A: Iron deficiency 
Explanation: 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