**Core Concept:** Carbon monoxide (CO) poisoning is a condition caused by the accumulation of CO in the bloodstream due to impaired oxygen transport and binding to hemoglobin. CO binds to hemoglobin with higher affinity compared to oxygen, forming carboxyhemoglobin (COHb), which decreases the availability of oxygen to tissues.

**Why the Correct Answer is Right:** In carbon monoxide poisoning, the body attempts to counteract the toxic effects of CO by shifting the oxygen dissociation curve to the left. This means that the hemoglobin will release oxygen more easily at a given PO2, leading to a decrease in PaCO2 (arterial partial pressure of carbon dioxide) as the body tries to compensate for the reduced oxygen-carrying capacity.

**Why Each Wrong Option is Incorrect:**

A) **Lt. shift of O2 dissociation curve**: This statement is partially correct but not the complete answer to the question. The correct answer involves the decrease in PaCO2 as well.
B) **↓ PaCO2**: This is also partially correct, as the correct answer should explain the compensatory mechanism in response to CO poisoning.
C) **↓ P50**: P50 is the partial pressure of oxygen at which hemoglobin releases 50% of its oxygen. In CO poisoning, P50 increases due to left shift of the oxygen dissociation curve, indicating a compensatory mechanism to counteract the toxic effects of CO.
D) **↓ O2 Content**: This statement does not address the primary mechanism of action of CO poisoning in the body or the compensatory response.

**Clinical Pearl:** The primary cause of CO poisoning is the binding of CO to hemoglobin, reducing its oxygen-carrying capacity. However, the body responds with a compensatory mechanism involving the left shift of the oxygen dissociation curve, which results in a decrease in PaCO2 (arterial partial pressure of carbon dioxide) as the body tries to maintain tissue oxygenation. This shift is due to the increased affinity of hemoglobin for oxygen, allowing for more efficient oxygen release in the tissues despite the decreased oxygen-carrying capacity.