**Core Concept**
The Hb-O2 dissociation curve illustrates the relationship between oxygen partial pressure and hemoglobin saturation in blood. A left shift in this curve indicates that hemoglobin has a higher affinity for oxygen, making it release oxygen less readily to tissues.

**Why the Correct Answer is Right**
A left shift in the Hb-O2 dissociation curve is primarily caused by an increase in 2,3-bisphosphoglycerate (2,3-BPG) levels or a decrease in CO2 or temperature, which all lead to an increase in oxygen affinity of hemoglobin. This is due to the Bohr effect, where a decrease in CO2 or an increase in pH (decrease in H+ ions) results in a decrease in the concentration of hydrogen ions, causing hemoglobin to release oxygen more readily.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as an increase in CO2 levels would cause a right shift in the Hb-O2 dissociation curve, not a left shift.

**Option B:** This option is incorrect as an increase in temperature would cause a right shift in the Hb-O2 dissociation curve, not a left shift.

**Option C:** This option is incorrect as an increase in pH (decrease in H+ ions) would cause a right shift in the Hb-O2 dissociation curve, not a left shift.

**Clinical Pearl / High-Yield Fact**
The Bohr effect is a critical concept in respiratory physiology, and understanding how changes in CO2, temperature, and pH affect oxygen affinity of hemoglobin is essential for managing patients with respiratory or cardiac disease.

**Correct Answer:** B. 2,3-Bisphosphoglycerate.