A 28-year-old primigravida from rural Maharashtra presents to the antenatal clinic at 16 weeks gestation. She has not taken any folic acid supplementation. Routine ultrasound shows a fetus with an open neural tube defect (spina bifida) at the lumbosacral region with exposed neural tissue. Maternal serum alpha-fetoprotein (AFP) is markedly elevated. What is the most appropriate immediate next step in management?