Final answer:
The most common heart defect in Down syndrome presents on auscultation as a heart murmur. This abnormal sound is caused by an opening in the interventricular septum, which is a characteristic of the Tetralogy of Fallot defect.
Step-by-step explanation:
Septal defects are commonly first detected through auscultation, listening to the chest using a stethoscope. In the case of the most common heart defect in Down syndrome, the presence of an unusual heart sound called a heart murmur may be detected. This abnormal sound is caused by an opening in the interventricular septum, which is a characteristic of the Tetralogy of Fallot defect. A heart murmur is typically confirmed by medical imaging, such as an echocardiogram.