## **Core Concept**
Carboxyhemoglobin (COHb) is a stable complex of carbon monoxide (CO) that binds to hemoglobin in the blood. The level of COHb is used as a marker of exposure to carbon monoxide. Normal levels of COHb are typically less than 5%.

## **Why the Correct Answer is Right**
A carboxyhemoglobin level greater than 5% indicates exposure to carbon monoxide, which could be due to various sources such as cigarette smoking, exhaust fumes, or smoke inhalation. Carbon monoxide has a high affinity for hemoglobin, forming carboxyhemoglobin and reducing the oxygen-carrying capacity of blood. This is significant because it can lead to tissue hypoxia. A level above 5% suggests significant exposure.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because less than 5% is considered within the normal range for carboxyhemoglobin levels, typically seen in non-smokers.
- **Option B:** This option might seem plausible but is not specified; however, without a specific value, it's hard to directly refute it based on the question alone.
- **Option C:** Similar to Option B, without a specific value, it's challenging to directly address its incorrectness based solely on the information given.
- **Option D:** This option is not provided, but presumably, it would be incorrect based on the context of the question and the correct answer provided.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that smokers often have elevated levels of carboxyhemoglobin due to carbon monoxide in cigarette smoke. This can affect the interpretation of COHb levels in smokers versus non-smokers. For instance, a level of 10% might be considered normal or expected in a heavy smoker but would be concerning in a non-smoker.

## **Correct Answer:** .