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: Physiology
[{'content': 'Which of the following is true regarding ideal alveolar ventilation- perfusion ratio? \nSelect and explain. \nA. Maximum at apex of lung, B. Minimum at apex of lung, C. Maximum at AV shunt, D. Maximum at base of lung', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. A. Maximum at apex of lunga. The ratio of pulmonary ventilation to pulmonary blood flow for the 'whole' lung at rest is about 0.8 (4.2 liter/minute ventilation divided by 5.5 liter/minute of blood flow).b. There are relatively marked differences in ventilation-perfusion ratio in various parts of the normal lung due to the effect of gravity, and local changes in the ventilation perfusion ratio are common in disease.c. Ventilation, as well as perfusion, in the upright position declines in a linear fashion from the bases to the apices of the lungs. However, the ventilation-perfusion ratios are high in the upper portions of the lungs.d. This high ventilation perfusion ratio at the apices account for the predilection for tuberculosis for this area of lung because the relatively high alveolar PO2 that results provides a favorable environment for the growth of tubercle bacilli", 'role': 'assistant'}]