A 72-year-old man on aspirin 100 mg daily for coronary artery disease presents with acute-onset bright red blood per rectum and hematochezia. He reports no abdominal pain, weight loss, or change in bowel habits. On digital rectal examination, there is bright red blood on the glove and a palpable mass 6 cm above the anal verge. Colonoscopy is performed and reveals a large, friable, ulcerated mass in the rectum with features suspicious for malignancy. What is the most appropriate immediate management after colonoscopic assessment?