A 38-year old woman presents to your clinic with chronic constipation and intermittent anal bleeding. During a rigid proctoscopy, a well-circumscribed ulcer is visualized on the anterior wall of the rectum, located at 8-cm from the anal verge. A biopsy is obtained, revealing minimal inflammation, with fibrosis of the lamina propria, and with a thickened muscularis mucosa. The most appropriate next step is