**Core Concept**
Carbon monoxide (CO) poisoning is a well-known cause of cherry-red postmortem staining due to the formation of carboxyhemoglobin. This condition occurs when CO binds to hemoglobin in red blood cells, leading to a decrease in oxygen delivery to tissues.

**Why the Correct Answer is Right**
When CO is inhaled, it binds to the iron in hemoglobin with a much higher affinity than oxygen, forming carboxyhemoglobin. This carboxyhemoglobin is a bright red color, which is visible in the skin and mucous membranes of victims of CO poisoning. The formation of carboxyhemoglobin is a key factor in the development of the characteristic cherry-red discoloration seen in postmortem examinations. The high affinity of CO for hemoglobin also explains why CO poisoning can occur at relatively low concentrations of CO in the air.

**Why Each Wrong Option is Incorrect**
* **Option A:** Cyanide poisoning typically causes a blue or purple discoloration of the skin and mucous membranes due to the formation of methemoglobin, which is a different type of hemoglobin.
* **Option B:** Hydrogen sulfide poisoning can cause a greenish discoloration of the skin and mucous membranes due to the formation of sulfmethemoglobin.
* **Option C:** Arsenic poisoning typically causes a pale or clammy appearance of the skin and mucous membranes due to its effects on the cardiovascular and nervous systems.

**Clinical Pearl / High-Yield Fact**
Cherry-red postmortem staining is a classic sign of CO poisoning, and it is often seen in conjunction with other symptoms such as headache, dizziness, and confusion. If you suspect CO poisoning in a patient, it is essential to have them evaluated promptly by a medical professional.

**Correct Answer: C. Arsenic poisoning typically causes a pale or clammy appearance of the skin and mucous membranes due to its effects on the cardiovascular and nervous systems.