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
Ribnotching of 4-9th ribs with double bulging is seen in - 
Select and explain. 
A. Aoic aneurysm, B. Aoic dessection, C. Co - arctation of aoa, D. Diaphragmatic hernia
C: Co - arctation of aoa
Answer: C: Co - arctation of aoa 
Explanation: Ans. is 'c' i.e., Coarctation of Aoa Radiological findings of COA 1. Abnormal Aoic arch : Commonestfinding Site of narrowing may be seen as an identation. Classic '3 sign' or double bulge sign' on left border of aoic 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 3'd or 4th ribs downwards. Rib notching is due to enlargement of intercostal aeries which act as collateral vessels. These collateral vessels produce indentation, on the ribs at the above sites.