Okay, let's tackle this question about the fatal dose of potassium cyanide. First, I need to recall the toxicology information regarding cyanide. Potassium cyanide is a highly toxic compound. The question is asking about the fatal dose, so I should remember the LD50 values for humans. I think the lethal dose is around 150-200 mg for adults. Let me verify that.

Wait, I remember that the oral LD50 for potassium cyanide in humans is approximately 150 mg. So if the options are in the range of 50-200 mg, then the correct answer would be the one around 150 mg. Now, the other options: maybe options include lower doses like 50 mg, which might not be fatal, or higher ones like 300 mg. Also, sometimes people confuse cyanide with other cyanide compounds. For example, sodium cyanide might have a slightly different toxicity profile, but potassium cyanide is more soluble and thus more toxic.

The core concept here is understanding the toxicity thresholds of cyanide compounds. Cyanide inhibits cytochrome c oxidase, disrupting cellular respiration. The fatal dose is related to the amount needed to overwhelm the body's ability to detoxify it. The correct answer is the one that aligns with established LD50 values. The incorrect options might be based on other cyanide forms, different routes of exposure, or confusion with other poisons.

Clinical pearl: Remember that even small amounts of cyanide can be lethal. The classic treatment involves nitrites and thiosulfate to counteract cyanide's effects. Also, high-yield fact: Potassium cyanide is more water-soluble than sodium cyanide, making it more rapidly absorbed and thus more immediately dangerous.

So, if the correct answer is 150-200 mg, then the options that are lower or higher would be incorrect. Let me structure the explanation with these points in mind.

**Core Concept**
Potassium cyanide is a potent cyanide compound that inhibits cytochrome c oxidase in the mitochondrial electron transport chain, causing cellular hypoxia. Its toxicity depends on solubility and absorption rate, with oral ingestion being the most common lethal route. The **LD50** (lethal dose for 50% of the population) is a key parameter in toxicology.

**Why the Correct Answer is Right**
The **oral LD50** for potassium cyanide in humans is approximately **150 mg**. This dose overwhelms the body's detoxification capacity via rhodanese, which converts cyanide to thiocyanate. Rapid onset of symptoms (e.g., dyspnea, seizures) occurs due to mitochondrial dysfunction. Solubility in water enhances absorption, making it more toxic than sodium cyanide in equivalent doses.

**Why Each Wrong Option is Incorrect**
**Option A:** A dose 250 mg is excessively high and contradicts established LD50 values; such doses would be uniformly lethal but are rarely encountered

✓ Correct Answer: D. 200 mg