A 24-year-old primigravida at 13 weeks gestation presents for her first antenatal ultrasound. She was not taking folic acid supplementation and has epilepsy treated with valproate 1500 mg daily. Transvaginal ultrasound shows a live singleton fetus with sagittal, coronal, and axial views of the fetal head demonstrating the finding marked **A** in the diagram. The fetal face, mandible, and base of skull are present, but the fetal brain tissue is replaced by a disorganized vascular mass exposed to amniotic fluid. Maternal serum alpha-fetoprotein is markedly elevated at 8.4 MOM. Which of the following is the most likely diagnosis and its key embryological basis?
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