What is the diagnosis?
Coarctation of the aorta.
If rib notching is only seen unilaterally on the left, what is likely present?
This suggests an associated aberrant subclavian artery arising after the coarctation. The right subclavian gives of the right internal mammary which links with the intercostals. As both subclavian and intercostals are after the coarctation there is no reason for collaterals to develop.
What is the appearance of the aortic contour known as?
Figure 3 sign.
Chest x-ray demonstrates a relatively small aortic knuckle with an indentation in its contour laterally. The heart size is within normal limits, although the left apex appears to point inferiorly, perhaps suggesting ventricular hypertrophy. Lungs and pleura are unremarkable. The inferior surface of multiple posterior ribs appears notched.