## Core Concept
Carbon monoxide (CO) toxicity is a critical condition that occurs when CO, a colorless, odorless gas, binds to hemoglobin in the blood, forming carboxyhemoglobin (COHb). This reduces the oxygen-carrying capacity of the blood and can lead to tissue hypoxia. CO also has direct toxic effects on cellular respiration.

## Why the Correct Answer is Right
The correct answer relates to understanding the effects and treatment of carbon monoxide poisoning. CO poisoning leads to tissue hypoxia due to its high affinity for hemoglobin, forming COHb. This shifts the oxygen-hemoglobin dissociation curve to the left, making it harder for tissues to release oxygen. CO also acts as a gaseous vasodilator and has direct toxic effects on cytochrome c oxidase in mitochondria, which can be lethal.

## Why Each Wrong Option is Incorrect
- **Option c) PO2 is decreased**: This statement might seem accurate because CO poisoning leads to decreased oxygen delivery to tissues. However, the partial pressure of oxygen (PO2) in the blood can actually be normal or even elevated because CO does not displace oxygen from hemoglobin in a way that directly lowers PO2; instead, it reduces oxygen delivery by forming COHb.
- **Option b) Treated by 5% CO2**: The treatment for CO poisoning primarily involves removing the patient from the source of CO, administering 100% oxygen (not 5% CO2), and in severe cases, hyperbaric oxygen therapy. The use of 5% CO2 is not a standard treatment.
- **Option d) Shift HbO2 dissociation curve to left**: This is true. CO binds to hemoglobin with high affinity, forming COHb, which shifts the oxygen-hemoglobin dissociation curve to the left. This shift decreases the release of oxygen to tissues.
- **Option e) Gaseous vasodilator**: This is true. CO is known to have vasodilatory effects.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the treatment for carbon monoxide poisoning involves immediate removal from exposure and administration of 100% oxygen. Hyperbaric oxygen therapy may be considered in severe cases. A critical clinical clue is that patients may have a cherry-red skin color due to the carboxyhemoglobin.

## Correct Answer: C. PO2 is decreased