A 69-year-old man is brought to the emergency department for new symptoms of confusion and sleep disturbance. He is not able to provide any history but his partner notes that he has cirrhosis due to chronic alcoholism She states that he has maintained abstinence from alcohol for the past 3 months. His medications include nadolol, furosemide, spironolactone, and lactulose.On examination, he looks jaundiced, the blood pressure is 102/78 mm Hg supine, and 86/64 mm Hg standing with an increase in heart rate from 72 to 100 beats/min. He is afebrile and the oxygen saturation is 98% on room air. The abdomen is soft; there is a palpable spleen tip and no evidence of ascites. He is not oriented to place or time and moves all four limbs on command. A digital rectal exam reveals dark black stool. Which of the following is the most likely cause?