**Core Concept**
The oxygen-hemoglobin dissociation curve is a graphical representation of the relationship between the partial pressure of oxygen (pO2) and the saturation of hemoglobin with oxygen (SO2). The curve is shifted to the left when hemoglobin has a higher affinity for oxygen, resulting in a lower p50 value.

**Why the Correct Answer is Right**
A left shift in the oxygen-hemoglobin dissociation curve is caused by an increase in 2,3-bisphosphoglycerate (2,3-BPG) levels, which binds to hemoglobin and reduces its affinity for oxygen. This is seen in conditions such as anemia, where the body attempts to compensate for the reduced oxygen-carrying capacity of the blood by increasing hemoglobin's affinity for oxygen. Other factors that can cause a left shift include increased CO2 levels, increased pH, and increased temperature.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because an increase in oxygen partial pressure (pO2) would actually cause a right shift in the curve, not a left shift.
* **Option B:** This option is incorrect because a decrease in 2,3-BPG levels would actually cause a right shift in the curve, not a left shift.
* **Option C:** This option is incorrect because a decrease in pH (acidosis) would actually cause a right shift in the curve, not a left shift.

**Clinical Pearl / High-Yield Fact**
Remember that a left shift in the oxygen-hemoglobin dissociation curve indicates that hemoglobin has a higher affinity for oxygen, which can be beneficial in conditions such as anemia but can also lead to tissue hypoxia in conditions where oxygen delivery is impaired.

**Correct Answer: C. 2,3-BPG levels are increased.**