**Core Concept**
The underlying principle being tested is the **oxygen carrying capacity** of blood, which is primarily determined by the amount of **hemoglobin** and its ability to bind **oxygen**. A decrease in oxygen carrying capacity with normal **partial pressure of oxygen (pO2)** in arterial blood suggests an issue with the blood's ability to transport oxygen, rather than a problem with oxygenation of the blood itself.

**Why the Correct Answer is Right**
Although the correct answer is not specified, a common cause of decreased oxygen carrying capacity with normal pO2 is **anemia**. In anemia, there is a decrease in the amount of hemoglobin or the number of red blood cells, leading to a reduced ability of the blood to carry oxygen. This can occur despite normal pO2 levels in arterial blood, as the issue lies in the blood's capacity to transport oxygen, not in the lungs' ability to oxygenate the blood.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific option provided, it's challenging to address each incorrect choice directly. However, incorrect options might include conditions that affect oxygenation (like respiratory diseases) rather than oxygen carrying capacity.
**Option B:** Similarly, without specifics, we can speculate that this option might be incorrect if it refers to a condition that does not directly impact hemoglobin or red blood cell count.
**Option C:** This could potentially be a condition that affects pO2 levels, which is not what the question is pointing towards.
**Option D:** This option might be related to a different aspect of oxygen transport or utilization that doesn't fit the specific scenario of decreased oxygen carrying capacity with normal pO2.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **anemia** can present with symptoms of decreased oxygen delivery to tissues (like fatigue, shortness of breath) even when pO2 levels are normal, highlighting the importance of assessing oxygen carrying capacity, not just oxygenation.

**Correct Answer:** Correct Answer: B. Anemia.