**Core Concept**
Post mortem luminescence refers to the phenomenon of a body glowing or emitting light after death. This is a rare but notable sign of certain types of poisoning. The underlying mechanism involves the breakdown of certain substances in the body that react with oxygen to produce light.

**Why the Correct Answer is Right**
The correct answer is **A. Hydrogen Cyanide (HCN)**. Hydrogen cyanide is a potent toxin that can cause rapid death. In cases of HCN poisoning, the body may exhibit post mortem luminescence due to the reaction of cyanide ions with oxygen in the presence of certain enzymes, particularly cytochrome c oxidase. This reaction leads to the production of light, which can be observed as a glowing or luminescent effect.

**Why Each Wrong Option is Incorrect**
* **Option B:** Arsenic poisoning does not typically cause post mortem luminescence. While arsenic can cause a range of symptoms and signs, including skin discoloration and nail changes, luminescence is not a characteristic feature.
* **Option C:** Organophosphate poisoning can cause a range of symptoms, including muscle weakness and respiratory failure, but it does not typically result in post mortem luminescence.
* **Option D:** Lead poisoning can cause a range of symptoms, including abdominal pain and constipation, but it does not typically result in post mortem luminescence.

**Clinical Pearl / High-Yield Fact**
In cases of suspected HCN poisoning, it's essential to note that post mortem luminescence can be a valuable clue in the diagnosis. However, it's worth noting that this sign is not unique to HCN poisoning and can be seen in other conditions, so a thorough investigation is always necessary to confirm the diagnosis.

**Correct Answer:** A. Hydrogen Cyanide (HCN)