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
The cardinal rule is that appearance of iron deficiency in an adult male is most often due to ...... until proven otherwise: 
Select and explain. 
A. Nutritional deficiency, B. Gastrointestinal blood loss, C. Renal disease, D. Malabsorption of iron due to worm infestation
B: Gastrointestinal blood loss
Answer: B: Gastrointestinal blood loss 
Explanation: 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