Ventricular septal defect Muscular ventricular septal defect 2012-04-08T09:43:15Z Ventricular septal defect, muscular Type 4 ventricular septal defect HP:0011623 Fyler:1320 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. UMLS:C0685707 SNOMEDCT_US:94706008