Final answer:
Semi-lobar holoprosencephaly is a brain abnormality where the cerebrum doesn't divide into two hemispheres properly, resulting in cognitive and physical symptoms.
Step-by-step explanation:
Semi-lobar holoprosencephaly is a developmental brain abnormality related to the incomplete separation of the prosencephalon, which is the embryonic forebrain. This condition results when the cerebrum fails to divide properly into two hemispheres, leading to moderate to severe cognitive and physical symptoms. The prosencephalon is initially a single structure that later divides to form distinct cerebral hemispheres, lobes, and deep structures like the corpus callosum which links both hemispheres allowing them to communicate. In holoprosencephaly, this division does not fully take place, causing defects in the formation of midline brain structures and facial anomalies. Semi-lobar holoprosencephaly lies on the spectrum of holoprosencephaly conditions, with a severity between alobar (most severe) and lobar (less severe) forms.