In the United States, what is the most likely underlying cause of iron deficiency anemia in an adult male or a postmenopausal female until proven otherwise?

Correct answer: Occult gastrointestinal bleeding

While dietary deficiency is common in developing countries, in adult males and postmenopausal females in developed nations, iron deficiency is almost always due to chronic blood loss. The gastrointestinal tract, specifically malignancy (like colon cancer), must be investigated as the primary source.