This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
In which of the following conditions left atrium is not enlarged: 
Select and explain. 
A. Ventricular septal defect, B. Atrial septal defect, C. Aoopulmonary window, D. Patent ductus aeriosus
B: Atrial septal defect
Answer: B: Atrial septal defect 
Explanation: Answer is B (ASD) ASD is associated with an enlarged right atrium & right ventricle to accommodate large volume of bloilL The left atrium is not enlarged because it decompresses itself by shunting blood to the right atrium at a minor difference in pressure. The left atrium may enlarge once Eisen menger's syndrome develops and a reversal of shunt is seen across the defect.