**Core Concept**
Potassium cyanide (KCN) is a highly toxic compound that acts by inhibiting cellular respiration, leading to asphyxiation of cells. It binds to the iron atom in cytochrome c oxidase, preventing the transfer of electrons and ultimately resulting in cellular death.

**Why the Correct Answer is Right**
The lethal dose of potassium cyanide is approximately 200 mg for an average adult, depending on factors such as body weight and individual susceptibility. This dose can cause rapid onset of symptoms, including respiratory failure, cardiac arrest, and ultimately, death. The mechanism of action involves the inhibition of cellular respiration, leading to a decrease in ATP production and an increase in lactic acid levels.

**Why Each Wrong Option is Incorrect**
**Option A:** 5 mg is an extremely low dose and would likely not be fatal, even in a susceptible individual.
**Option B:** 10 mg is still a relatively low dose and would not be sufficient to cause lethal cyanide poisoning.
**Option C:** 20 mg is closer to the lethal dose but is still significantly lower than the actual fatal dose of 200 mg.

**Clinical Pearl / High-Yield Fact**
Cyanide poisoning can be rapidly reversed with the administration of hydroxocobalamin, a cyanide antidote that binds to cyanide ions and forms a non-toxic complex.

**✓ Correct Answer: D. 200 mg**