A 35-year-old woman presents with progressive headaches and blurred vision for 3 months. MRI brain shows a well-circumscribed, intra-axial lesion in the pineal region with heterogeneous signal intensity, T1 hypointense, T2 hyperintense, with patchy enhancement post-gadolinium. There is mild hydrocephalus. CSF analysis shows elevated protein (120 mg/dL) and normal glucose. What is the most appropriate next step in management?