<prompt> Most serious injury is atopen pneumothoraxdiaphragmatic injuryFlail chestSingle rib fracture  <response>: Answer- A. open pneumothoraxThis is a large open defect in the chest 3 cm, leading to equilibration between intrathoracic and atmospheric pressure.Air accumulates in the hemithorax rather than in the lung with each inspiration, leading to profound hypoventilation on the affected side and hypoxia. 