Describe the effects of Ventilation/Perfusion (V/Q) inequality on the partial pressure of oxygen (PaO2) in arterial blood.
Overall answers lacked sufficient detail on a core area of respiratory physiology. Answers expected included a description of V/Q ratios throughout the lungs and an explanation of how V/Q inequality lowers PaO2.
For a more helpful set of college comments for what is an undoubtedly important and often-repeated question, the candidates are directed to Question 5 from the second paper of 2014 and Question 6 from the first paper of 2008. Both of those asked for the effects of V/Q mismatch on both gases, but this time in 2017 the college decided to limit their question to oxygen. This, perhaps, is because the pass rate for the abovementioned questions was 8% and 0%, respectively.
A good diagram to produce in such a question comes from West (1977):
Specifically, if one were ever also asked about CO2 removal, one could use this graph to demonstrate the different effects of dropping the V/Q from 1.0 to 0.1 on the absorption of oxygen and removal of CO2:
West, John B. "Ventilation-perfusion relationships." American review of respiratory disease 116.5 (1977): 919-943.
Petersson, Johan, and Robb W. Glenny. "Gas exchange and ventilation–perfusion relationships in the lung." (2014): 1023-1041.