A 4-year-old girl from Mumbai presents with recurrent respiratory tract infections and mild dyspnea on exertion. Cardiac examination reveals a systolic ejection murmur at the right upper sternal border with a fixed, widely split S2. Echocardiography shows an ostium secundum atrial septal defect (ASD). Her pediatrician mentions that this differs significantly from coarctation of the aorta. Which finding best discriminates ASD from coarctation of the aorta?