Failure of the callosal fibers to form a normal connection may be partial or complete and is associated with a dilated third ventricle and outward angling of the horns. What is this condition called?

• A. Agenesis of corpus callosum
• B. Hydranencephaly
• C. Arachnoid cyst
• D. Acrania

Answer: (A)

When the fibers that connect the two cerebral hemispheres fail to form a normal bridge, the brain develops agenesis or hypogenesis of the corpus callosum. Without this major commissure, the ventricular system changes: the third ventricle becomes dilated and the lateral ventricles’ horns angle outward because there is no corpus callosum to keep the hemispheres aligned. This combination of a partial or complete absence of the corpus callosum with a dilated third ventricle and outwardly angled horns is characteristic of agenesis of the corpus callosum. It can occur in isolation or with other brain malformations and may be seen on ultrasound or MRI. Other conditions described—such as hydranencephaly, arachnoid cyst, or acrania—have different patterns of brain involvement and ventricle appearance, not the specific callosal absence and ventricle configuration seen here.