A 28-year-old pregnant woman at 17 weeks of gestation undergoes routine prenatal screening, which includes maternal serum markers. Her maternal serum alpha-fetoprotein (MSAFP) level is significantly elevated (> 2.5 MoM). A targeted ultrasound shows an intact fetal skull and cerebral hemispheres, with a lumbosacral cystic sac and splayed posterior vertebral elements. The fetal head shows a "lemon sign" and the cerebellum has a "banana sign". Which of the following is the most likely fetal condition?