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: Anatomy
All of the following structures press over the oesophagus, EXCEPT? 
Select and explain. 
A. Aoic arch, B. Left atrium, C. Left bronchus, D. Right bronchus
D: Right bronchus
Answer: D: Right bronchus 
Explanation: Anteriorly the normal esophagus is indended from above downwards by three impoant structures that cross it, the arch of aoa, left bronchus and left atrium. Various structures that may press on the oesophagus and cause obstruction are:Enlarged left atriumBronchial carcinomaEnlarged lymph nodes at the hilum of lungDilated aoaCongenital abnormalities of the main thoracic aeriesThe 4 constrictions of the oesophagus are:First constriction is at the pharyngoesophageal junctionSecond constriction is produced by the arch of aoa as it crosses the anterior and lateral aspects of oesophagusThird constriction is produced by the left bronchus as it crosses the oesophagusFouh constriction is at the oesophageal opening in the diaphragm.