A 67-year-old man presents with recurrent passage of bright-red blood and clots. After initial fluid replacement, his blood pressure is 126/74 mmHg and pulse is 92 beats/minute, but he continues to pass approximately 250 mL of blood every 10 minutes and his haemoglobin is 86 g/L (130–170 g/L). He has no abdominal pain, previous inflammatory bowel disease or recent colonoscopy. Which one of the following is the most appropriate investigation to localise the active bleeding?