**Core Concept**
Nitrites, such as amyl nitrate, are used in the treatment of cyanide poisoning due to their ability to form methemoglobin, which has a higher affinity for cyanide than cytochrome c oxidase. This forms a stable complex with cyanide, preventing it from inhibiting cellular respiration.

**Why the Correct Answer is Right**
Amyl nitrate works by inducing a rapid increase in methemoglobin levels, which then binds to cyanide, forming a stable cyanomethemoglobin complex. This complex is unable to bind to cytochrome c oxidase and thus does not inhibit cellular respiration. The rapid increase in methemoglobin levels is crucial in the treatment of cyanide poisoning, as it allows for the quick formation of the cyanomethemoglobin complex. The use of amyl nitrate in cyanide poisoning is a classic example of the principle of "chemical antidotes" in toxicology.

**Why Each Wrong Option is Incorrect**
**Option A:** Sodium nitrite is actually used in conjunction with amyl nitrate in the treatment of cyanide poisoning, as it helps to form methemoglobin. However, amyl nitrate is the correct answer in this context.
**Option B:** Succimer is a chelating agent used in the treatment of lead poisoning, not cyanide poisoning.
**Option C:** Activated charcoal is used to treat a wide range of poisonings by absorbing the toxin in the gastrointestinal tract, but it is not specific to cyanide poisoning.
**Option D:** Fomepizole is an antidote for methanol and ethylene glycol poisoning, not cyanide poisoning.

**Clinical Pearl / High-Yield Fact**
Remember that amyl nitrate is used in conjunction with sodium nitrite to treat cyanide poisoning, and that the rapid formation of methemoglobin is critical in this context.

**Correct Answer: C. Cyanide poisoning.**