The most common atrial septal defect (ASD) seen clinically is

B. The most common ASD is foramen secundum defect, which is caused by excessive resorption of the septum primum or the septum secundum. This results in an opening between the atria (patent foramen ovale). Some of these defects may remain undiagnosed and may be tolerated for a long time (up to age 30 years before the person presents clinically).