In assessing a newborn, which cardiac defect is most likely indicated by an inability to palpate femoral pulses?

The inability to palpate femoral pulses in a newborn is highly indicative of coarctation of the aorta. This condition involves a narrowing of the aorta, typically occurring just distal to the left subclavian artery. As a result, blood flow to the lower part of the body, including the legs, can be significantly reduced. This reduction is what leads to weak or absent femoral pulses, while upper body pulses remain strong.

In contrast, the other cardiac conditions listed do not typically present with absent femoral pulses. Patent ductus arteriosus may lead to increased blood flow to the lungs and might not affect the tamponade of femoral pulses. Ventricular septal defect and atrial septal defect, while they may present with other symptoms and complications, do not have the same direct impact on the aortic flow leading to diminished femoral pulses. Thus, when assessing for cardiac defects in a newborn, the absence of femoral pulses strongly suggests coarctation of the aorta.