**Core Concept**
Carbon monoxide (CO) poisoning is a common occupational hazard for firefighters, causing headache, dizziness, and confusion. CO binds to hemoglobin with high affinity, forming carboxyhemoglobin (COHb), which reduces oxygen delivery to tissues.

**Why the Correct Answer is Right**
The patient's symptoms and normal arterial blood gas suggest CO poisoning. The best first step in management is administering 100% oxygen to displace CO from hemoglobin and reduce COHb levels. This increases the half-life of CO, allowing it to be exhaled more quickly. High concentrations of oxygen are essential to effectively displace CO from hemoglobin.

**Why Each Wrong Option is Incorrect**
**Option A:** Administering activated charcoal would be ineffective in treating CO poisoning, as it does not bind CO. Activated charcoal is primarily used for oral ingestion of toxic substances.
**Option B:** Using a pulse oximeter may not accurately detect CO poisoning, as it only measures oxygen saturation, not COHb levels. A carboxyhemoglobin (COHb) level should be measured to confirm the diagnosis.
**Option C:** The patient's normal arterial blood gas does not rule out CO poisoning, as CO poisoning can occur without respiratory symptoms or abnormalities in blood gas values.

**Clinical Pearl / High-Yield Fact**
The " cherry-red" color of the skin and mucous membranes is a classic sign of CO poisoning, due to the presence of carboxyhemoglobin.

**Correct Answer:** C. Administer 100% oxygen.