This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System Pa 2, Subject name: Physiology
An increase in which of the following increases the O2 affinity of hemoglobin? 
Select and explain. 
A. Temperature, B. PCO2, C. H+ concentration, D. Carbon monoxide added to the blood
D: Carbon monoxide added to the blood
Answer: D: Carbon monoxide added to the blood 
Explanation: A small amount of carbon monoxide added to blood increases its oxygen affinity, that is, it causes a leftward shift of the O2 dissociation curve . All the other choices reduce the oxygen affinity of hemoglobin, that is, they shift the dissociation curve to the right.