We report the case of a 67-year-old male with a history of arterial hypertension and atrial fibrillation, who was admitted for acute renal failure (creatinine: 4.41 mg/dl) and hypotension. He also presented hyponatremia (129 mmol/L) and hypokalemia (2.7 mmol/L). The patient referred profuse diarrhea during the previous two months as a possible triggering cause. Physical examination showed signs of dehydration and palpation of a polypoid mass in the rectal ampulla.