**Core Concept**
Potassium cyanide (KCN) is a highly toxic compound that acts by inhibiting cellular respiration through the disruption of cytochrome c oxidase in the mitochondria. This leads to a rapid depletion of ATP, causing cellular dysfunction and ultimately death.

**Why the Correct Answer is Right**
The fatal dose of potassium cyanide is approximately 200-300 mg for an adult human, which is extremely low compared to other toxins. This is because KCN has a high affinity for the cytochrome c oxidase enzyme, leading to rapid inhibition of cellular respiration. The mechanism of action involves the formation of cyanide ions (CN-) that bind to the iron atom in the enzyme, preventing the transfer of electrons and ultimately halting the production of ATP.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely incorrect because it is not a commonly reported fatal dose of KCN.
* **Option B:** This option is incorrect because it is not supported by scientific evidence regarding the toxicity of KCN.
* **Option C:** This option is incorrect because it is not a recognized standard for the fatal dose of KCN.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the fatal dose of KCN can vary depending on the route of exposure, individual tolerance, and other factors. However, the generally accepted range of 200-300 mg remains a crucial piece of information for medical professionals to diagnose and treat cyanide poisoning.

**Correct Answer: D. 200-300 mg**