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
Most common cause of idiopathic interstitial pneumonia is 
Select and explain. 
A. Sarcoidosis, B. Organizing pneumonia, C. Idiopathic pulmonary fibrosis, D. Lipoid pneumonia
C: Idiopathic pulmonary fibrosis
Answer: C: Idiopathic pulmonary fibrosis 
Explanation: Idiopathic pulmonary fibrosis is the most common form of idiopathic interstitial pneumonia.It is characterised by exeional dyspnea,non productive cough,inspiratory crackles with or without digital clubbing.HRCT show patchy,predominantly basilar,subpleural reticular opacities,traction bronchiectasis and honeycombing.Pulmonary function tests often reveal a restrictive pattern. Reference:Harrison' s medicine-18th edition,page no:2165.