**Core Concept**
Atrial septal defects (ASDs) are characterized by an abnormal opening in the interatrial septum, allowing blood to flow between the left and right atria. Ostium secundum type of ASD is the most common type, accounting for approximately 70% of all ASDs. This type of defect occurs in the fossa ovalis, a region of the interatrial septum that is normally closed by the foramen ovale during fetal development.

**Why the Correct Answer is Right**
In ostium secundum type of ASD, the abnormal opening in the interatrial septum allows blood to flow from the left to the right atrium due to the higher pressure in the left atrium. This results in increased flow through the pulmonary circuit, leading to symptoms such as palpitations, dyspnea, and fatigue. The increased flow also leads to right ventricular enlargement and eventual pulmonary hypertension.

**Why Each Wrong Option is Incorrect**
* **Option A:** Symptoms of ostium secundum type of ASD typically begin in childhood or adolescence, not in adulthood.
* **Option B:** While palpitations and dyspnea are common symptoms, syncope is not typically associated with this type of ASD.
* **Option C:** While pulmonary hypertension can occur in long-standing cases, it is not a characteristic feature of ostium secundum type of ASD.

**Clinical Pearl / High-Yield Fact**
A key feature of ostium secundum type of ASD is the presence of a characteristic "fixed splitting" of the second heart sound (S2), which is due to the increased flow through the pulmonary valve.

**Correct Answer:** B. Syncope.