Final answer:
The Left atrium is not typically enlarged in an atrial septal defect (ASD). ASD causes an abnormal opening in the interatrial septum, usually resulting in the enlargement of the right chambers of the heart and the main pulmonary artery due to increased blood flow to the right side, not to the left chambers.
Step-by-step explanation:
The cardiac chambers that are not typically enlarged in the case of an atrial septal defect (ASD) are A. the Left atrium and D. the Left ventricle. An atrial septal defect is a congenital anomaly in which there is an abnormal opening in the interatrial septum, allowing blood to flow between the two atria. Due to this defect, the right atrium and right ventricle may receive excessive blood flow, which can lead to their enlargement. The main pulmonary artery can also become enlarged as more blood is shunted into the lungs. However, the left atrium and left ventricle are not typically enlarged as a direct result of an ASD because the defect usually leads to a left-to-right shunt, increasing flow toward the right side of the heart.
The chamber that initially receives blood from the systemic circuit is the right atrium. The systemic circuit carries deoxygenated blood from the body back to the heart, and this blood enters the heart through the right atrium. Following this, in the typical cardiac circulation pattern established after birth, the interatrial septum should have no openings; the foramen ovale closes and forms the fossa ovalis.
Therefore, the correct answer to the original question, regarding which chamber is not typically enlarged due to an atrial septal defect, is A. the Left atrium.