The MAC line is a reference line, drawn on a sagittal midline image through mammillary body and anterior commisure, intersecting the corpus callosum. It indicates the anatomic transition from genu to body of corpus callosum, and helps to distinguish the normal from the abnormal genu.
In this case, note the foreshortened genu (cf an aged matched normal) and absent rostrum, indicating a subtle callosal dysgenesis that was not easily appreciated on prenatal imaging. Also note the abnormally close positioning of the fornix to corpus callosum, indicating that the CSP between them was also abnormally formed.