What causes melena?

• A. Oxidation of iron in hemoglobin
• B. Bleeding from lower GI tract
• C. Ingestion of iron supplements
• D. Infection with Giardia

Answer: (A)

Melena is the dark, tarry stool that results when blood from the GI tract is digested. The key process is the oxidation of iron in hemoglobin as blood passes through the stomach and intestines, forming dark compounds (like hematin) that give the stool its characteristic black color. This is why oxidation of iron in hemoglobin best explains the appearance of melena and its association with upper GI bleeding.

Bleeding from the lower GI tract usually produces bright red or maroon stools because the blood hasn’t had time to be digested. Ingestion of iron supplements can darken stools as well, but that darkness comes from the iron itself, not from digested blood. Giardia infection alters stool color and consistency in a different way, typically causing malabsorption with pale, bulky stools, not tarry melena.