**Core Concept**
The concept of rightward shift in the oxygen-hemoglobin dissociation curve refers to the increased affinity of hemoglobin for oxygen, making it more difficult for oxygen to be released to tissues. This is a critical physiological adaptation to ensure adequate oxygen delivery during periods of high metabolic demand or low oxygen availability.

**Why the Correct Answer is Right**
A rightward shift in the oxygen-hemoglobin dissociation curve is typically caused by factors that increase 2,3-bisphosphoglycerate (2,3-BPG) levels or decrease pH, temperature, or CO2 levels. These factors include increased metabolic rate, high altitude, and certain disease states such as chronic lung disease. The correct answer option is likely to be a factor that does not cause a rightward shift.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is a cause of rightward shift, as it increases 2,3-BPG levels, which in turn increases hemoglobin's affinity for oxygen.
* **Option B:** This option also causes a rightward shift by decreasing pH, which increases hemoglobin's affinity for oxygen.
* **Option D:** This option is a cause of rightward shift, as it decreases CO2 levels, which in turn decreases the Bohr effect and increases hemoglobin's affinity for oxygen.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that a rightward shift in the oxygen-hemoglobin dissociation curve is a compensatory mechanism to ensure adequate oxygen delivery during periods of high metabolic demand or low oxygen availability.

**Correct Answer: A. Increased 2,3-BPG levels are not a cause of rightward shift, as they actually increase hemoglobin's affinity for oxygen, not decrease it.**