**Core Concept**
The most common congenital cardiac defect is a structural abnormality present at birth, affecting the heart's development during embryogenesis. It is a leading cause of morbidity and mortality in children and adults.

**Why the Correct Answer is Right**
The most common congenital cardiac defect is **atrial septal defect (ASD)**, specifically the ostium secundum type. This defect occurs due to the failure of the foramen secundum to close during fetal development, resulting in an abnormal communication between the left and right atria. This communication allows blood to flow between the two atria, leading to increased volume load on the right heart and potential complications such as pulmonary hypertension.

**Why Each Wrong Option is Incorrect**
* **Option A:** Ventricular septal defect (VSD) is a significant congenital cardiac defect, but it is less common than ASD.
* **Option B:** Tetralogy of Fallot is a cyanotic congenital heart defect, but it is not the most common type.
* **Option D:** Patent ductus arteriosus (PDA) is a congenital heart defect, but it is less common than ASD.

**Clinical Pearl / High-Yield Fact**
The most common congenital cardiac defect, ASD, often presents with symptoms such as dyspnea, fatigue, and palpitations, but may be asymptomatic in early childhood.

**Correct Answer: C. Atrial septal defect (ASD).**