The answer is D: lobar collapse secondary to mucus plugging. The endotracheal tube (ETT) is in a satisfactory position but the left lower lobe (LLL) is collapsed and can be seen as a ‘triangular’ shaped density behind the heart (dotted line figure 3 indicates the hypotenuse of the triangle). Because the LLL is no longer aerated and has effectively become solid, there is loss of the normal outline of the left hemi-diaphragm, which is usually seen because of its interface with …