Most common type of atrial septal defect is ___________
**Core Concept**
Atrial septal defects (ASDs) are congenital heart defects characterized by an abnormal opening in the interatrial septum, allowing blood to flow between the left and right atria. The most common type of ASD is determined by its location and size.
**Why the Correct Answer is Right**
The most common type of ASD is the **ostium secundum** type, accounting for approximately 70% of all cases. This type of ASD is located in the inferior part of the interatrial septum, near the fossa ovalis. It is usually a solitary defect and is often associated with a patent foramen ovale. The ostium secundum ASD is typically caused by a failure of closure of the foramen ovale during fetal development.
**Why Each Wrong Option is Incorrect**
* **Option A:** **Sinus venosus** ASDs are less common and typically located near the entrance of the superior vena cava or inferior vena cava into the right atrium.
* **Option B:** **Atrial septal defects** of the **ostium primum** type are also less common and typically associated with other cardiac anomalies, such as ventricular septal defects or atrioventricular canal defects.
* **Option C:** **Coronary sinus** ASDs are rare and involve an abnormal connection between the coronary sinus and the left atrium.
**Clinical Pearl / High-Yield Fact**
The **Eustachian valve**, a remnant of the embryonic valve of the inferior vena cava, is often associated with ostium secundum ASDs and can be seen on echocardiography or cardiac catheterization.
**Correct Answer: C. Atrial septal defects of the sinus venosus type are less common and typically located near the entrance of the superior vena cava or inferior vena cava into the right atrium.