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
Transport of carbon monoxide (CO) is diffusion limited because 
Select and explain. 
A. High affinity of CO for haemoglobin, B. Alveolar membrane is less permeable to CO, C. CO crosses epithelial barrier slowly, D. On exposure to air there is sudden increase in partial pressure
A: High affinity of CO for haemoglobin
Answer: A: High affinity of CO for haemoglobin 
Explanation: Since CO binds to Hb very rapidly, there is always a pressure gradient for its diffusion. 
There is no free CO in blood. CO diffusion can be increased only if the thickness of the capillary decreases or if the area of the capillary increases. 
So transport of CO is diffusion limited.