The cardinal rule is that appearance of iron deficiency in an adult male is most often due to ...... until proven otherwise:
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
Nutritional deficiency
Gastrointestinal blood loss
Renal disease
Malabsorption of iron due to worm infestation