**Core Concept**
The O2 - Hb dissociation curve represents the relationship between the partial pressure of oxygen and the saturation of hemoglobin with oxygen. A shift to the left indicates an increased affinity of hemoglobin for oxygen. This can occur due to various factors affecting the binding properties of hemoglobin.

**Why the Correct Answer is Right**
Although the specific correct answer is not provided, factors that typically cause a left shift of the O2 - Hb dissociation curve include decreased CO2, decreased temperature, and decreased 2,3-bisphosphoglycerate (2,3-BPG). These conditions increase hemoglobin's affinity for oxygen, making it less willing to release oxygen to tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific answer choices, it's challenging to address each option directly. However, generally, a decrease in pH (acidosis) or an increase in CO2 would shift the curve to the right, not the left.
**Option B:** Similarly, without specifics, an increase in 2,3-BPG or temperature would also shift the curve to the right.
**Option C:** And any factor that decreases hemoglobin's affinity for oxygen would shift the curve to the right.
**Option D:** This would also be incorrect if it represents a condition that decreases oxygen affinity.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that conditions or factors that shift the O2 - Hb dissociation curve to the left will increase the affinity of hemoglobin for oxygen but may impair oxygen delivery to tissues.

**Correct Answer:** Correct Answer: D. Decreased 2,3-Bisphosphoglycerate.