A 45-year-old woman presents to the clinic with a 4-month history of progressively worsening low back pain, particularly with sitting. Rectal examination reveals a palpable posterior retrorectal mass 5 cm from the anal verge. Flexible sigmoidoscopy reveals a bulge in the posterior rectum, but normal overlying mucosa. Magnetic resonance imaging reveals a 4.5-cm well-circumscribed, solid mass in the presacral space, concerning for a neurogenic tumor. Which of the following is the most appropriate route of biopsy?