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: Physiology
Maximum alveolar aerial oxygen difference is seen in: 
Select and explain. 
A. Pulmonary embolism, B. Severe asthma, C. Interstitial lung disease, D. Foreign body in upper airway
A: Pulmonary embolism
Answer: A: Pulmonary embolism 
Explanation: Ans: A. Pulmonary embolism (Ref: Ganong 25/e p634, 24/e p634)In pulmonary embolism gradient increases to very high level (I0 to 83 mm Hg from normal level of 5 mm Hg).But normal Pa02 level on aerial blood gas analysis does not exclude the diagnosis of acute pulmonary embolism.It is also increased in ILD.