A 32-year-old woman from Delhi, who did not receive folic acid supplementation in the periconceptional period, delivers a male infant at 38 weeks. The newborn has a small, skin-covered bulge in the lumbosacral midline with normal lower limb motor and sensory function and normal sphincter control. Imaging shows a fluid-filled sac containing only meninges and cerebrospinal fluid, with the spinal cord in normal anatomical position. What is the embryological defect responsible for this condition?