**Core Concept**
The septum primum atrial septal defect (ASD) is a congenital heart defect characterized by an abnormal opening in the atrial septum, which is the wall of tissue separating the left and right atria. The overburden in ASD refers to the increased volume of blood flowing through one of the atria, which can lead to enlargement and dysfunction of that chamber.

**Why the Correct Answer is Right**
In the septum primum type of ASD, the defect is located in the upper part of the interatrial septum, near the fossa ovalis. This defect allows blood to flow from the left atrium to the right atrium, resulting in overburden in the right atrium. The increased volume of blood flowing through the right atrium can lead to right atrial enlargement and dysfunction. The right ventricle may also be affected, leading to right ventricular enlargement and dysfunction.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the overburden in septum primum type of ASD occurs in the right atrium, not the left atrium.
* **Option B:** This option is incorrect because the overburden in septum primum type of ASD does not occur in the left ventricle.
* **Option D:** This option is incorrect because the overburden in septum primum type of ASD does not occur in the right ventricle, although the right ventricle may be affected due to increased volume of blood flowing through the right atrium.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the location and size of the ASD defect can affect the chamber that is overburdened. In general, defects located in the lower part of the interatrial septum, such as the ostium secundum type of ASD, tend to result in overburden in the right atrium, while defects located in the upper part of the interatrial septum, such as the septum primum type of ASD, tend to result in overburden in the left atrium.

**Correct Answer: C. Right atrium.**