A 30 - year - old male chronic smoker presents with progressive breathlessness for 1 month. Chest x - ray is shown. What is the most probable diagnosis?

* The given chest X-ray shows a bulla occupying 1/3rd of the hemithorax.it is characteristic of Idiopathic giant bullous emphysema which is also known "Vanishing Lung syndrome". It is common in young male and in smokers. It usually bilateral, asymmetrical and commonly involves the upper lobes of the lungs. * In pneumothorax, the lung will be collapsed towards the hilum and visceral pleural reflection is seen.* Visceral pleural reflection helps us to differentiate between pneumothorax and vanishing lung syndrome.