A 28-year-old primigravida from rural Maharashtra presents at 18 weeks of gestation for antenatal screening. Ultrasound reveals a large defect in the lumbosacral region with herniation of meninges, cerebrospinal fluid, and neural tissue through the vertebral defect. The fetus shows associated lower limb paralysis and loss of sphincter control. Maternal serum alpha-fetoprotein (AFP) is markedly elevated. What is the most likely diagnosis?