What condition is defined by anencephalus?

Anencephalus is a severe neural tube defect characterized by the congenital absence of significant portions of the brain and skull. This condition occurs when the neural tube, which develops into the central nervous system, fails to close completely during the early stages of fetal development. As a result, infants born with anencephalus typically lack a major part of the brain, particularly the cerebral hemispheres, and often have an underdeveloped or absent skull.

The other choices do not pertain to anencephalus. A congenital heart defect refers to abnormalities in the heart structure that are present at birth. Developmental delay relates to a slower-than-expected rate of development in areas such as language, motor skills, or cognitive functions. Partial hearing loss denotes reduced auditory sensitivity, which is not connected to the structural anomalies associated with anencephalus. Hence, the correct definition aligns closely with option B, capturing the essence of this serious condition.