Most common type of Atrial septal defect is
**Core Concept**
The most common type of atrial septal defect (ASD) is related to the **ostium secundum**, which is a defect in the region of the fossa ovalis. This type of defect is due to an abnormality in the formation of the septum secundum. ASDs are characterized by an opening in the interatrial septum, allowing blood to flow between the left and right atria.
**Why the Correct Answer is Right**
The correct answer is related to the **ostium secundum** type, which accounts for about 75% of all ASDs. This type of defect occurs due to a failure of the septum secundum to fully cover the ostium secundum during fetal development, resulting in a persistent opening. The **ostium secundum** defect is typically located in the area of the fossa ovalis.
**Why Each Wrong Option is Incorrect**
**Option A:** Is incorrect because it does not specify the correct location or type of ASD.
**Option B:** Is incorrect because it is not a common type of ASD.
**Option C:** Is incorrect because, although it is a type of ASD, it is not the most common.
**Option D:** Is incorrect because it is also not the most common type of ASD.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that **ostium secundum** ASDs are often associated with other cardiac anomalies, such as mitral valve prolapse.
**Correct Answer:** Correct Answer: C. Ostium Secundum type.