**Core Concept**
Carboxyhemoglobin (COHb) is a stable complex formed between carbon monoxide (CO) and hemoglobin (Hb). This complex reduces the oxygen-carrying capacity of blood, leading to tissue hypoxia. The formation of COHb is a critical consideration in cases of suspected carbon monoxide poisoning.

**Why the Correct Answer is Right**
The percentage of carboxyhemoglobin in the blood is a key indicator of carbon monoxide exposure. A carboxyhemoglobin level of more than 5% is generally considered a threshold for significant clinical concern. This is because CO binds to hemoglobin with an affinity 210 times greater than oxygen, effectively reducing the amount of oxygen that can be transported to tissues. The presence of COHb can be detected through laboratory tests, which measure the proportion of hemoglobin bound to carbon monoxide.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because carboxyhemoglobin levels are not directly correlated with the severity of symptoms or outcome in carbon monoxide poisoning.
* **Option B:** This option is incorrect because the presence of carboxyhemoglobin is a specific indicator of carbon monoxide exposure, not a general marker of respiratory distress.
* **Option C:** This option is incorrect because while carbon monoxide poisoning can cause cardiovascular effects, carboxyhemoglobin levels are not a direct measure of cardiovascular risk.

**Clinical Pearl / High-Yield Fact**
In cases of suspected carbon monoxide poisoning, a high index of suspicion is warranted, even in the absence of overt symptoms. A carboxyhemoglobin level above 5% is a critical threshold for initiating treatment and ensuring proper patient care.

**Correct Answer:** B.