**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. This can lead to increased flow through the pulmonary circuit, causing various clinical symptoms.

**Why the Correct Answer is Right**
The most common type of ASD is the **Secundum ASD**, accounting for approximately 70% of all ASDs. This type of defect occurs in the middle of the septum, near the fossa ovalis, and is often associated with a patent foramen ovale (PFO). The Secundum ASD is typically diagnosed during childhood or adolescence, although it may not be symptomatic until later in life.

**Why Each Wrong Option is Incorrect**
**Option A:** Ostium primum ASD - This type of ASD is located near the inferior portion of the interatrial septum and is often associated with other congenital heart defects, such as ventricular septal defects (VSDs) and atrioventricular septal defects (AVSDs).
**Option B:** Sinus venosus ASD - This type of ASD is located near the superior vena cava and is often associated with abnormalities of the pulmonary veins. It is less common than the Secundum ASD.
**Option C:** Coronary sinus ASD - This type of ASD is located near the coronary sinus and is often associated with abnormalities of the coronary sinus and the cardiac veins.

**Clinical Pearl / High-Yield Fact**
The Secundum ASD is often asymptomatic in childhood, but may present with symptoms such as dyspnea, fatigue, and palpitations during adolescence or adulthood.

**Correct Answer: A. Secundum ASD**