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
Alveolar-aerial gradient is increased in all except 
Select and explain. 
A. Diffusion defects, B. Right to left shunt, C. Chronic bronchitis, D. Interstitial lung disease
C: Chronic bronchitis
Answer: C: Chronic bronchitis 
Explanation: The normal alveolar aerial gradient is 5-15mmHg. In chronic bronchitis there is alveolar hypoventilation so alveolar aerial gradient is normal.