A 32-year-old woman from Delhi delivers a male infant at 38 weeks gestation. Postnatal examination reveals a midline lump at the lower lumbar region with a tuft of hair overlying it, and the infant has mild lower limb weakness and diminished anal reflex. Ultrasound of the lumbosacral region shows a fluid-filled sac tethering the spinal cord. The infant is otherwise hemodynamically stable with no respiratory distress. What is the most appropriate immediate next step in management?