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
An increase in which of the following parameters will shift the O2 dissociation curve to the left - 
Select and explain. 
A. Increased temperature, B. Increased partial pressure of CO2, C. Increased 2, 3 DPG concentration, D. Increased oxygen affinity of hemoglobin
D: Increased oxygen affinity of hemoglobin
Answer: D: Increased oxygen affinity of hemoglobin 
Explanation: Hb-O2 dissociation curve shift to left indicates more affinity of Hb to O2 and thus less delivery of O2 to tissues.
Shift to left seen in
-        Increased pH
-        Decreased 2,3 DPG, temperature, PCO2
-        HbF (Fetal)
Shift to right seen in
-        Decreased pH
-        Increased 2,3DPG, temperature, PCO2
-        HbS (Sickle cell)