A 58-year-old man with established alcohol-related cirrhosis presents with two episodes of black, offensive stool and light-headedness. His blood pressure is 108/68 mmHg, he is alert, and there is no fever or abdominal tenderness; urgent endoscopy is being arranged. Although his liver disease is classified as Child–Pugh class A and he has no clinical evidence of infection, which one of the following is the most appropriate next step?