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: Surgery
Which of the following is the cause of sudden and unexpected onset of dyspnea at rest: September 2011 
Select and explain. 
A. Severe anemia, B. Spontaneous pneumothorax, C. COPD, D. Large pleural effusion
B: Spontaneous pneumothorax
Answer: B: Spontaneous pneumothorax 
Explanation: Ans. B: Spontaneous pneumothorax Sudden and unexpected dyspneic episodes at rest can be associated with pulmonary emboli, spontaneous pneumothorax, hypercapnea secondary to breath holding, or anxiety Pneumothorax: Presence of gas in pleural space Sudden dyspnea followed by cough and chest pain is seen Primary Spontaneous pneumothorax: occurs without antecedent history of trauma to thorax Common in males/ smokers - Rupture of subpleural bleb is a common cause Secondary spontaneous pneumothorax occurs in presence of underlying lung pathology A traumatic pneumothorax results from penetrating or non-penetrating injury Intrapleural pressure is equal to atmospheric pressure in open pneumothorax