A 1-day-old infant was born with the vault of the skull undeveloped, leaving the malformed brain exposed. A diagnosis of exencephaly is made. What is the embryologic cause of this condition?

The rostral neuropore closes during the fourth week of development. If this does not occur, the forebrain primordium is abnormal and the calvaria or vault fails to develop. Toxoplasmosis infection during embryologic development leads to microcephaly, in which the brain and calvaria are small in size. These patients usually have mental retardation due to an undeveloped brain. An ossification defect in the bones of the skull is often a result of hydrocephalus. Caudal displacement of the cerebellar structures would not lead to an undeveloped calvaria or vault. Maternal alcohol abuse leads to intrauterine growth restriction, causing microcephaly and mental retardation.