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
Ventilation-perfusion ratio is maximum at 
Select and explain. 
A. Apex of lung, B. Base of lung, C. Posterior lobe of lung, D. Middle of the lung
A: Apex of lung
Answer: A: Apex of lung 
Explanation: Apical alveoli have low ventilation and perfusion per unit volume of the lung, whereas the basal alveoli have high ventilation and perfusion.
Ventilation-perfusion ratio and the PaO2 is maximum at the apex. PCO2 is maximum at the base.