Your patient takes 5 mg of warfarin (Coumadin) daily and reports having black, tarry stool today. What do you most likely suspect?

The presentation of black, tarry stools, known as melena, is most consistent with gastrointestinal bleeding. Warfarin is an anticoagulant that increases the risk of bleeding by inhibiting the synthesis of vitamin K-dependent clotting factors. When a patient on warfarin reports such symptoms, it raises a red flag for possible upper gastrointestinal bleeding, as the dark coloration indicates that blood has been digested before being excreted.

In this scenario, while hemorrhoids, constipation, and dehydration may affect bowel movements, they do not typically present with melena. Hemorrhoids usually result in bright red blood on the surface of the stool rather than tarry stools. Constipation is characterized by infrequent or difficult bowel movements rather than the color or consistency described. Dehydration does not cause melena directly but can lead to other complications.

Thus, the combination of the patient’s anticoagulation therapy and the symptom of black, tarry stools strongly indicates that gastrointestinal bleeding is the most likely condition to suspect.