What characteristic is typically observed on a chest X-ray for older children with coarctation of the aorta?

In older children with coarctation of the aorta, the chest X-ray often reveals the "3" sign, which is a specific finding that suggests the presence of this condition. The "3" sign appears as a contour of the aorta that resembles the number three due to the narrowing at the site of coarctation and the dilation of the aorta proximal and distal to the constriction. As the blood pressure increases in these segments due to the obstruction, it leads to characteristic reshaping of the aorta, which can be visualized on X-ray.

This finding is important as it helps in diagnosing coarctation of the aorta, especially when considering differential diagnoses for chest X-ray in children.