**Core Concept**
Potassium cyanide (KCN) is a highly toxic compound that acts by inhibiting cellular respiration, leading to cellular asphyxiation and ultimately, death. This occurs through the inhibition of cytochrome c oxidase, a key enzyme in the electron transport chain.

**Why the Correct Answer is Right**
The fatal dose of KCN is typically around 200-300 mg for an adult, which is extremely small due to its high toxicity. This is because KCN causes a rapid decrease in cellular ATP production, leading to the disruption of cellular functions, including the inhibition of oxidative phosphorylation. The mechanism involves the binding of KCN to the iron atom in cytochrome c oxidase, which prevents the transfer of electrons and ultimately halts cellular respiration.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is not the correct dose of KCN that is fatal. However, it may be worth noting that a dose of 50 mg is often cited as the minimum lethal dose for KCN.
* **Option B:** This option is incorrect because it is not the correct dose of KCN that is fatal. A dose of 1,000 mg is far more than the minimum lethal dose and is likely to be fatal.
* **Option C:** This option is incorrect because it is not the correct dose of KCN that is fatal. A dose of 500 mg is still above the minimum lethal dose and would likely be fatal.

**Clinical Pearl / High-Yield Fact**
One key point to remember is that KCN can be absorbed through the skin, mucous membranes, and gastrointestinal tract, making it extremely hazardous to handle or ingest.

**Correct Answer: B. 200-300 mg for an adult.**