Which of the following is the most common site of origin for an atrial septal defect (ASD)?

Explanation

## Atrial Septal Defects: Classification and Frequency **Key Point:** Ostium secundum ASD accounts for approximately 70–75% of all ASDs, making it the most common type by far. ### Embryological Basis The atrial septum develops from multiple structures: 1. **Septum Primum** — grows from endocardial cushions toward the foramen ovale 2. **Septum Secundum** — muscular fold that grows from the roof of the common atrium 3. **Foramen Ovale** — normal opening between septa during fetal life ### Types of ASD and Their Frequency | ASD Type | Embryological Defect | Frequency | Key Features | |----------|----------------------|-----------|---------------| | **Ostium Secundum** | Inadequate growth of septum secundum; excessive resorption of septum primum | 70–75% | Defect in region of foramen ovale; usually low-risk hemodynamically | | **Ostium Primum** | Failure of endocardial cushions to fuse | 15–20% | Defect in lower atrial septum; often associated with cleft mitral valve | | **Sinus Venosus** | Abnormal development of sinus venosus; defect in wall between SVC/IVC and atrium | 10–15% | Often associated with partial anomalous pulmonary venous return (APVR) | | **Coronary Sinus** | Unroofing of coronary sinus | <1% | Rare; defect between coronary sinus and left atrium | **High-Yield:** Ostium secundum ASD is the **most common congenital heart defect** in adults and the **most frequent ASD type** overall. ### Clinical Pearl **Ostium secundum ASDs** are often diagnosed late in life because they may be hemodynamically well-tolerated initially. The defect lies in the region of the foramen ovale and involves inadequate development of the septum secundum combined with excessive resorption of the septum primum. ### Mnemonic **"OSSC"** — **O**stium **S**ecundum is the **S**ingle **C**ommonest ASD (70–75% of cases). **Warning:** Do not confuse ostium secundum (most common) with ostium primum (second most common, 15–20%). Primum defects are associated with endocardial cushion defects and cleft mitral valve, whereas secundum defects are isolated septal defects.