This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Medicine
[{'content': 'Commonest etiological factor in acute exacerbations of COPD is \nSelect and explain. \nA. Viral infections, B. Bacterial infections, C. Paiculate air pollutants, D. Idiopathic', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Exacerbations are a prominent feature of the natural history of COPD. Exacerbations are episodes of increased dyspnea and cough and change in the amount and character of sputum.Studies suggest that acquiring a new strain of bacteria is associated with increased near-term risk of exacerbation and that bacterial infection/superinfection is involved in over 50% of exacerbations. Viral respiratory infections are present in approximately one-third of COPD exacerbations.In a significant minority of instances (20-35%), no specific precipitant can be identifiedRef: Harrison 19e pg: 1707', 'role': 'assistant'}]