DAVID M. MARTIN, MD / SCIENCE PHOTO LIBRARY DAVID M. MARTIN, MD / SCIENCE PHOTO LIBRARY
Stained colon. Endoscope view of a dark patch (upper right) in the colon, called melanosis coli, caused by prolonged use of laxative drugs to treat constipation. The drugs contained anthracene, a pigment extracted from coal tar. Melanosis coli is usually asymptomatic and harmless.
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