The trabecular septum is the largest part of the interventricular septum. It extends from the membranous septum to the apex and superiorly to the infundibular septum. A defect in the trabecular septum is called muscular VSD if the defect is completely rimmed by muscle. The phenotype is HP_0011623 (Muscular ventricular septal defect). Also known as: Type 4 ventricular septal defect.