One of the most common causes of blood in the stool is an anal fistula. This condition causes a small tunnel between the anus and the skin near the anus. This fistula is usually a complication of a rectal abscess, a pocket of puss in the nearby tissue. As a result of this opening, some people develop blood in the stool. If the source of the bleeding is near the anus or in the lower part of the digestive tract, the blood is usually bright red, fresh blood. However, if the stool appears dark brown or black, the source of bleeding is likely from the upper part of the digestive tract -- the stomach.
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