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
[{'content': 'Ventilation-perfusion ratio is maximum at \nSelect and explain. \nA. Apex of lung, B. Base of lung, C. Posterior lobe of lung, D. Middle of the lung', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Apical alveoli have low ventilation and perfusion per unit volume of the lung, whereas the basal alveoli have high ventilation and perfusion.\nVentilation-perfusion ratio and the PaO2 is maximum at the apex. PCO2 is maximum at the base.', 'role': 'assistant'}]