ACP InternistMKSAP Quiz: Evaluation for Intermittent, Bright Red Blood per Rectum

A 51-year-old man presents with a 10-week history of intermittent bright red blood per rectum. His medical history is unremarkable, and he takes no medications. Physical examination reveals normal vital signs and a normal digital rectal examination. Fecal occult blood is positive. A colonoscopy identifies a nonobstructing lesion located 10 cm from the anal verge, while the rest of the colon appears normal. MRI scan results indicate that the lesion is invading into the muscularis but has not breached the full thickness, and there are no abnormal lymph nodes detected. Additionally, a contrast-enhanced CT scan of the chest and abdomen shows no signs of metastases. Biopsy results confirm the presence of adenocarcinoma. What is the most appropriate treatment for this patient?