Until the seventh week, there is a crescent-shaped ________ between the free edge of the interventricular septum and the fused endocardial cushions.

• A. Interventricular Foramen
• B. Primum Foramen
• C. Secundum Foramen
• D. AV Cushion Defect

Answer: (A)

During ventricular septation, an opening sits at the edge where the developing muscular interventricular septum meets the fused endocardial cushions. This crescent-shaped gap is the interventricular foramen. It remains open until about the seventh week, when the membranous portion of the interventricular septum forms and the endocardial cushions fuse, closing the foramen and completing separation of the ventricles. The other foramina mentioned are in the atrial septum (foramen primum and foramen secundum) and describe different facets of septation, not this ventricular region. An AV cushion defect represents a malformation in the AV canal area, not the normal interventricular foramen.