A 30-year-old primigravid woman at 18 weeks' gestation comes to the physician for a prenatal visit. Physical examination shows a uterus consistent in size with an 18-week gestation. Uitrasonography shows several intracranial anomalies, including a single ventricle, small cerebrum, fused thalami, and genesis of the corpus callosum. These findings were most likely caused by the failure of which of the following structures to differentiate?
A) Diencephalon
B) Mesencephalon
C) Prosencephalon
D) Rhombencephalon
E) Telencephalon