This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
Rib Notching of 4–9th ribs with double bulging is seen in – 
Select and explain. 
A. Aortic aneurysm, B. Aortic dessection, C. Coarctation of aorta, D. Diaphragmatic hernia
C: Coarctation of aorta
Answer: C: Coarctation of aorta 
Explanation: Radiological findings of COA
1. Abnormal Aortic arch : Commonest finding
Site of narrowing may be seen as an identation.
Classic '3 sign' or double bulge sign' on left border of aortic shadow is formed by (above downwards)

Prestenotic dilatation
Coarctation itself (indentation)


Poststenotic dilatation

2. Rib notching : is a frequent sign in adults. It is not seen until late childhood.
It is seen on the inferior margin of posterior halves of ribs from 3n1 or 4th ribs downwards. Rib notching is due to enlargement of intercostal arteries which act as collateral vessels. These collateral vessels produce indentation, on the ribs at the above sites.