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': 'The cardinal rule is that appearance of iron deficiency in an adult male is most often due to ...... until proven otherwise: \nSelect and explain. \nA. Nutritional deficiency, B. Gastrointestinal blood loss, C. Renal disease, D. Malabsorption of iron due to worm infestation', 'role': 'user'}, {'content': 'Answer: B \nExplanation: The cardinal rule is that the appearance of iron deficiency in an adult male means gastrointestinal blood loss. Reference: Harrisons Principles of Internal Medicine, 18th Edition, Page 846', 'role': 'assistant'}]