**Core Concept**
The O2 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 curve is influenced by various factors, including pH, temperature, and 2,3-bisphosphoglycerate (2,3-BPG) levels.

**Why the Correct Answer is Right**
Although the correct answer is not specified, a decrease in temperature, a decrease in 2,3-BPG, or an increase in pH would shift the curve to the left. For instance, a decrease in 2,3-BPG reduces the binding of 2,3-BPG to hemoglobin, which increases hemoglobin's affinity for oxygen.

**Why Each Wrong Option is Incorrect**
**Option A:** An increase in this parameter is not specified, but if it were related to factors like temperature or 2,3-BPG, an increase would typically shift the curve to the right, not left.
**Option B:** Similarly, without knowing the parameter, we can say that factors increasing oxygen affinity (like decreased 2,3-BPG) shift the curve left, while others (like increased CO2) shift it right.
**Option C:** This option is also unspecified but would be incorrect if it involved an increase in a parameter that normally shifts the curve to the right, such as an increase in CO2 or temperature.
**Option D:** Without the parameter, we can only say that factors decreasing hemoglobin's affinity for oxygen (thus shifting the curve to the right) would be incorrect.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that conditions or factors that increase the affinity of hemoglobin for oxygen (shifting the curve to the left) include decreased CO2, decreased temperature, and decreased 2,3-BPG.

**Correct Answer:** Correct Answer: D. Decrease in 2,3-BPG.