<prompt> The cardinal rule is that appearance of iron deficiency in an adult male is most often due to ... until proven otherwise:Nutritional deficiencyGastrointestinal blood lossRenal diseaseMalabsorption of iron due to worm infestation  <response>: 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 