A 45-year-old man presents to the emergency department with acute onset of bilateral lower limb pain and weakness following heavy lifting 6 hours ago. On examination, he has saddle anesthesia (perianal and perineal sensory loss), urinary retention with post-void residual of 350 mL, and reduced anal sphincter tone. An urgent sagittal MRI lumbar spine is obtained. The structure marked **B** in the diagram shows compression from a large central disc herniation at L4-L5. Which of the following is the most appropriate immediate management?