What is a neural tube defect characterized by a lack of closure at the base of the brain?

Anencephaly is a neural tube defect that specifically occurs when there is incomplete closure of the neural tube at the cranial end, leading to a significant portion of the brain and skull not forming properly. This condition results in the absence of a major part of the brain, typically the cerebrum, and can critically impact neurological function and overall viability of the infant. Infants diagnosed with anencephaly are often stillborn or die shortly after birth due to the severity of the brain malformations and associated complications.

Spina bifida, on the other hand, refers to a defect involving the lower section of the neural tube, resulting in incomplete closure of the spinal column. Hydrocephalus is a condition characterized by an accumulation of cerebrospinal fluid within the ventricles of the brain, which can arise due to different underlying issues but is not a defect of the neural tube itself. Encephalopathy refers to a general term for any brain disease, damage, or malfunction, rather than a specific structural defect related to the neural tube closure. Thus, the defining characteristics of anencephaly clearly delineate it as the correct answer to this question regarding neural tube defects.