A 65-year-old woman presents with a presacral mass incidentally identified on a lumbar spine magnetic resonance imaging scan. The patient reports back pain but denies any constitutional or gastrointestinal symptoms. On digital rectal examination, the mass is not palpable. Her most recent colonoscopy was 2 years ago and was negative for intraluminal pathology. Imaging reveals a 4-cm cystic lesion located above the S4 level (Figure). What is the best next step in management?