Which congenital heart defect is the second most common and typically asymptomatic until adulthood?

The atrial septal defect (ASD) is indeed the second most common congenital heart defect and is often asymptomatic during childhood, with symptoms typically not manifesting until adulthood. An ASD allows for a left-to-right shunt of blood, which means that oxygenated blood from the left atrium flows into the right atrium instead of going into the systemic circulation.

In early life, this condition may not present any noticeable symptoms because the heart and lungs can usually compensate for the added blood flow to the right heart chambers and lungs. Over time, however, the increased volume load can lead to complications such as right atrial and ventricular dilation, pulmonary hypertension, and ultimately heart failure or arrhythmias in adulthood if left untreated.

This condition may remain undiagnosed for many years until routine examinations or echocardiographic studies reveal it or until complications develop, which aligns with the question's focus on the asymptomatic nature of the defect typically delaying its diagnosis until later in life.