**Core Concept**
The question is testing the ability to recognize postmortem changes associated with specific types of poisoning. Greenish-blue discoloration is a characteristic postmortem sign that can be linked to certain toxins. This phenomenon is related to the accumulation of specific compounds within the body, which alter the normal coloration of the skin.

**Why the Correct Answer is Right**
The greenish-blue discoloration is typically seen in cases of **Cyanide poisoning**. This occurs due to the formation of methemoglobin, which is a stable form of hemoglobin that cannot bind oxygen. Methemoglobin has a blue-green color, which is responsible for the characteristic discoloration seen in cyanide poisoning. Cyanide inhibits the cytochrome c oxidase enzyme in the electron transport chain, leading to cellular asphyxiation and the production of methemoglobin.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely to be incorrect as it does not match any common postmortem changes associated with poisoning. Without more information, it's difficult to determine what this option might represent.
* **Option B:** This option might be a distractor related to other types of poisoning, such as carbon monoxide poisoning, which typically causes a cherry-red discoloration due to the formation of carboxyhemoglobin.
* **Option D:** This option could be a distractor related to other types of poisoning, such as opiate overdose, which typically causes a pale or mottled appearance due to respiratory depression and decreased oxygenation.

**Clinical Pearl / High-Yield Fact**
In cases of cyanide poisoning, the presence of a greenish-blue discoloration can be a valuable diagnostic clue. However, it's essential to note that this sign can be variable and may not always be present. A high index of suspicion and thorough toxicology screening are necessary to confirm the diagnosis.

**Correct Answer:** C. Cyanide poisoning.