<prompt> The normal narrowing in the middle of the oesophagus is caused byAzygos veinHemiazygos veinRight main stem bronchusLeft main stem bronchus  <response>: Middle constriction of oesophagus is due to left bronchus .27.5cm11 inch from insisor teeth. BD CHAURASIA;S HUMAN ANATOMY VOLUME 1, 6TH Edition . page no 282, fig 20.7 