**Core Concept**
Ostium primum atrial septal defect (ASD) is a congenital heart defect characterized by an abnormal opening in the interatrial septum, specifically in the lower part of the septum near the atrioventricular valves. This defect allows blood to flow between the left and right atria.

**Why the Correct Answer is Right**
In ostium primum ASD, the septal defect is located near the atrioventricular valves, typically involving the septal leaflet of the mitral valve and the septal leaflet of the tricuspid valve. This results in left-to-right shunting of blood, leading to increased volume load on the right heart. The defect is often associated with other congenital heart defects, such as mitral valve abnormalities and tricuspid valve abnormalities.

**Why Each Wrong Option is Incorrect**
**Option A:** Ostium secundum ASD is a different type of congenital heart defect characterized by an abnormal opening in the fossa ovalis, not in the lower part of the septum near the atrioventricular valves.
**Option B:** Sinus venosus ASD is a rare type of congenital heart defect characterized by an abnormal opening in the upper part of the septum near the superior vena cava, not in the lower part of the septum near the atrioventricular valves.
**Option C:** Ventricular septal defect (VSD) is a different type of congenital heart defect characterized by an abnormal opening in the interventricular septum, not in the interatrial septum.

**Clinical Pearl / High-Yield Fact**
Ostium primum ASD is often associated with other congenital heart defects, such as mitral valve abnormalities and tricuspid valve abnormalities, and may require surgical repair to prevent complications such as atrial arrhythmias and heart failure.

**Correct Answer:** D (assuming D is the correct option, as it was not provided)