A decrease in which of the following parameters will shift the O2 dissociation curve to the right?

**Core Concept**
The O2 dissociation curve represents the relationship between the partial pressure of oxygen (pO2) and the saturation of hemoglobin with oxygen. The curve shifts in response to various physiological and pathological conditions, influencing the delivery of oxygen to tissues.

**Why the Correct Answer is Right**
A decrease in pH (acidosis) shifts the O2 dissociation curve to the right, indicating that hemoglobin has a lower affinity for oxygen. This is because the Bohr effect describes how acidic conditions increase the release of oxygen from hemoglobin to the tissues. The decrease in pH leads to a conformational change in hemoglobin, resulting in a reduced binding affinity for oxygen and an increased release of oxygen to the tissues. This is mediated by the histidine residues on the hemoglobin molecule, which are protonated in acidic conditions, promoting the release of oxygen.

**Why Each Wrong Option is Incorrect**
**Option B:** A decrease in the partial pressure of CO2 (hypocapnia) actually shifts the O2 dissociation curve to the left, not the right. This is because the Bohr effect is reversed in hypocapnic conditions, and hemoglobin has a higher affinity for oxygen.

**Option C:** An increase in 2,3-DPG concentration (not a decrease) shifts the O2 dissociation curve to the right. This is because 2,3-DPG binds to hemoglobin and decreases its affinity for oxygen, promoting the release of oxygen to the tissues.

**Option D:** An increase in temperature shifts the O2 dissociation curve to the right, not a decrease. This is because higher temperatures increase the kinetic energy of the hemoglobin molecule, reducing its affinity for oxygen and promoting the release of oxygen to the tissues.

**Clinical Pearl / High-Yield Fact**
Remember the Bohr effect: acidosis (decreased pH) shifts the O2 dissociation curve to the right, increasing the release of oxygen to the tissues. This is a critical concept in understanding the regulation of oxygen delivery to tissues in various physiological and pathological conditions.

**✓ Correct Answer: A. pH**