**Core Concept**
The patient's low oxygen saturation in the setting of third-degree burns is likely due to the release of carbon monoxide (CO) from the fire, which binds to hemoglobin with a higher affinity than oxygen. This results in a condition known as carbon monoxide poisoning, where the body's tissues are not receiving adequate oxygen.

**Why the Correct Answer is Right**
Carbon monoxide binds to the iron atom in the heme group of hemoglobin, forming carboxyhemoglobin. This reduces the amount of hemoglobin available for oxygen binding, leading to a decrease in oxygen delivery to tissues. In this case, the patient's arterial oxygen saturation is 50%, which is significantly lower than expected given the arterial PO2 of 95 mm Hg. This discrepancy suggests that the patient's hemoglobin is bound to carbon monoxide, rather than oxygen.

**Why Each Wrong Option is Incorrect**
* **Option A:** Hypoxemia caused by third-degree burns is not typically due to impaired gas exchange, as the patient's respiratory rate is only slightly elevated and their arterial PO2 is normal.
* **Option B:** Carbon dioxide poisoning would not typically cause a decrease in oxygen saturation, as it does not bind to hemoglobin.
* **Option D:** Methemoglobinemia is a condition where hemoglobin is oxidized to its ferric state, but it is not typically caused by third-degree burns.

**Clinical Pearl / High-Yield Fact**
In cases of suspected carbon monoxide poisoning, it is essential to obtain a carboxyhemoglobin level, as the patient's oxygen saturation may be normal despite significant impairment of oxygen delivery to tissues.

**Correct Answer: C. Carbon monoxide poisoning**