## Core Concept
Hemoglobin is a crucial protein in red blood cells that carries oxygen from the lungs to the body's tissues. It is composed of four polypeptide chains (globins) and four heme groups, which bind to oxygen. Understanding the structure and function of hemoglobin is essential for appreciating various physiological and pathological processes.

## Why the Correct Answer is Right
The correct answer, **D. Hemoglobin has a higher affinity for CO than O2**, highlights a critical property of hemoglobin. Hemoglobin's affinity for carbon monoxide (CO) is significantly higher than its affinity for oxygen (O2), which is why CO poisoning can be so dangerous. This high affinity is due to the strong binding of CO to the heme group, forming carboxyhemoglobin and reducing the amount of oxygen delivered to tissues.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but any statement contradicting basic hemoglobin facts would be incorrect.
- **Option B:** Similarly, without the specific content of Option B, we can't directly address its inaccuracy, but it would likely misrepresent hemoglobin's characteristics.
- **Option C:** Again, lacking the details of Option C, we can infer that it would not accurately describe a true statement about hemoglobin.

## Clinical Pearl / High-Yield Fact
A key clinical point to remember is that **smokers have increased levels of carboxyhemoglobin** due to the CO produced by cigarette smoke. This can lead to a decrease in the oxygen-carrying capacity of the blood and contribute to various health issues, including cardiovascular disease.

## Correct Answer Line
**Correct Answer: D. Hemoglobin has a higher affinity for CO than O2**