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: Anaesthesia
If an anaesthetist at high altitude uses plenum vaporizers. It will deliver the vapour concentration – 
Select and explain. 
A. Higher than the original concentration at high partial pressure, B. Lower than the concentration at lower partial pressure, C. Higher than the concentration at same partial pressure, D. Lower than concentration at same partial pressure
C: Higher than the concentration at same partial pressure
Answer: C: Higher than the concentration at same partial pressure 
Explanation: Plenum vaporizer is typically set to deliver a 1-2 kPa partial pressure of anaesthetic agent. At sea level, where atmospheric pressure is around 100 kPa, this equates conveniently to a concentration of 1-2% of anaesthetic agent (1-2kPa out of 100 kPa).
At higher altitude, the atmospheric pressure is low (e.g. 75 kPa). The plenum vaporizer deliver 1-2 kPa partial pressure of anaesthetic agent. So, the concentration of anaesthetic vapours will 1.25 - 2.5% → Greater than the concentration of anaesthetic agent at same partial pressure at sea level.