Cherry red colours of mucous membranes are seen in which of the following poisons:
**Core Concept**
Carbon monoxide poisoning is a condition where exposure to carbon monoxide (CO) leads to tissue hypoxia due to the formation of carboxyhemoglobin (COHb), which reduces the oxygen-carrying capacity of hemoglobin. The cherry red discoloration of mucous membranes and skin is a classic sign of carbon monoxide poisoning.
**Why the Correct Answer is Right**
The formation of carboxyhemoglobin occurs when carbon monoxide binds to the iron atom in the heme group of hemoglobin, forming a stable complex. This binding is stronger than the binding of oxygen, resulting in a left shift of the oxygen-hemoglobin dissociation curve and a decrease in the oxygen-carrying capacity of the blood. The cherry red coloration is due to the presence of carboxyhemoglobin, which absorbs light in the blue-violet end of the spectrum, giving tissues a reddish hue.
**Why Each Wrong Option is Incorrect**
**Option A:** Cyanide poisoning causes a different type of tissue hypoxia, where the cytochrome c oxidase enzyme is inhibited, preventing the electron transport chain from functioning. This results in a bluish discoloration of the skin and mucous membranes, not cherry red.
**Option B:** Methanol poisoning leads to metabolic acidosis and visual disturbances, but it does not cause a cherry red coloration of mucous membranes.
**Option C:** Organophosphate poisoning causes cholinergic toxicity, leading to muscarinic and nicotinic receptor overstimulation, but it does not cause a characteristic change in mucous membrane color.
**Clinical Pearl / High-Yield Fact**
The cherry red coloration of mucous membranes in carbon monoxide poisoning is a classic sign that should prompt immediate action to remove the patient from the source of carbon monoxide exposure and provide supplemental oxygen.
**Correct Answer:** A. Carbon monoxide.