**Core Concept**
The oxygen-hemoglobin dissociation curve is a graphical representation of the relationship between the partial pressure of oxygen and the saturation of hemoglobin with oxygen. A rightward shift of the curve indicates a decrease in the affinity of hemoglobin for oxygen, which facilitates the release of oxygen to tissues.

**Why the Correct Answer is Right**
Decreased CO2 (option B) is not a cause of a rightward shift of the oxygen-hemoglobin dissociation curve. In fact, increased CO2 (hypercapnia) causes a rightward shift by decreasing pH and increasing temperature, both of which decrease the affinity of hemoglobin for oxygen. On the other hand, **decreased CO2** would decrease the concentration of carbonic acid, which would lead to an increase in pH (alkalosis) and a leftward shift of the curve.

**Why Each Wrong Option is Incorrect**
* **Option A:** Increased hydrogen ions (decreased pH) cause a rightward shift of the oxygen-hemoglobin dissociation curve by decreasing the affinity of hemoglobin for oxygen.
* **Option C:** Increased temperature also causes a rightward shift of the curve by decreasing the affinity of hemoglobin for oxygen.
* **Option D:** Increased 2,3-Bisphosphoglycerate (BPG) causes a rightward shift of the curve by decreasing the affinity of hemoglobin for oxygen.

**Clinical Pearl / High-Yield Fact**
Remember that factors that decrease the affinity of hemoglobin for oxygen (rightward shift) are those that are beneficial for tissue oxygenation, such as increased temperature, increased CO2, and increased BPG, whereas factors that increase affinity (leftward shift) are those that are beneficial for maintaining oxygen stores, such as decreased temperature, decreased CO2, and decreased 2,3-BPG.

**✓ Correct Answer: B. Decreased CO2**