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: Radiology
[{'content': 'Bilateral rib notching is seen in – \nSelect and explain. \nA. Coarctation of aorta, B. PDA, C. TAPVC, D. All of the above', 'role': 'user'}, {'content': "Answer: A \nExplanation: Radiological features of COA\n\nReverse figure of '3' sign or double bulge sign or E sign on the left border of aortic shadow is formed by (above downward:- Prestenotic dilatation; coarctation itself (indentation); poststenotic dilatation.\nDouble aortic knuckle.\nDock's sign:- There is bilateral notching of inferior margins of 3-8 ribs. Rib notching is absent in the 1st & 2nd ribs because first and second intercostal arteries do not take part in the collateral circulation. Rib notching increases with age and after the age of 7 years, rib notching is present in almost all the cases.", 'role': 'assistant'}]