<prompt> Most common type of Atrial septal defect isOstium primumOstium secundumCushion defectEndocardial hyperophy  <response>: The atrial septal defect can occur in any poion of the atrial septumsecundum, primum or sinus venosusOSTIUM SECUNDUM:Most common type of ASD accounts for 7 of al congenital hea disease.Defect situated at fossa ovalis.Associated with structurally normal atrioventricular AVvalves.OSTIUM PRIMUM ENDOCARDIAL CUSHION DEFECT:Defect situated inferior to fossa ovalis.Associated with the cleft in AV valves.Most common cardiac lesion in Down;s syndrome is endocardial cushion defect. 