This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System, Subject name: Radiology
Mounier Kuhn syndrome is seen on imaging as? 
Select and explain. 
A. Tracheal stenosis, B. Tracheobronchomegaly, C. Tracheal erosions, D. Sabre sheath trachea
B: Tracheobronchomegaly
Answer: B: Tracheobronchomegaly 
Explanation: Ans BIt is a disorder of elastic cartilage and smooth muscle of trachea where there is unusual enlargement of the trachea and bronchus during inspiration. Biphasic CT with images of the trachea obtained during inspiration and expiration - is most sensitive investigation. Two-phase chest radiographs will also demonstrate the enlargement of the trachea on inspiration and collapse during expiration, but they are clearly less sensitive. To consider the diagnosis, the diameter of the trachea should be greater than 3 cm: this is usually measured 2 cm above the aortic arch