A 22-year-old primigravida had a fetal screening ultrasound study at 18 weeks showing a single large cerebral ventricle and fused thalami. On physical examination at birth at 36 weeks' gestation, the infant is small for gestational age and has multiple anomalies, including postaxial polydactyly of hands and feet, cyclopia, microcephaly, cleft lip and palate, and rocker-bottom feet. The infant dies 1 hour after birth. Which of the following CNS abnormalities best explains these findings?

Holoprosencephaly is a midline defect in which there is absent (alobar) or partial (semilobar) cerebral hemispheric development. It can occur in trisomy 13, as in this case, with other midline defects. It also may be seen in cases of maternal diabetes mellitus. Anencephaly is the absence of a fetal cranial vault, which leads to the absence of most of the brain. Arnold-Chiari II malformation results in a small posterior fossa, a misshapen midline cerebellum with downward displacement of the vermis, and tenting of the tectal plate. Dandy-Walker malformation is characterized by aplasia or hypoplasia of the cerebellar vermis, cystic enlargement of the fourth ventricle, and hydrocephalus. A periventricular leukomalacia is a form of perinatal injury that is caused by hypoxic-ischemic events or infections.