A 71-year-old woman with a 4-year history of lumbar spinal stenosis presents with progressive bilateral leg pain and paresthesias on walking. She reports relief when sitting or leaning forward. MRI shows central canal stenosis at L4–L5 with ligamentum flavum hypertrophy and facet joint arthropathy. Conservative management with NSAIDs, physical therapy, and epidural steroid injections has provided only temporary relief over the past 18 months. She is now a candidate for surgical intervention. Which of the following surgical approaches would be most appropriate for this patient, and why?