## Core Concept
Sodium nitroprusside is a medication used in certain cases of severe hypertension and for controlled hypotension. However, one of its notable side effects is the release of cyanide ions. In the context of cyanide poisoning, understanding its mechanism and treatment is crucial.

## Why the Correct Answer is Right
The correct mechanism involves the formation of **methemoglobin**, which has a higher affinity for cyanide than cytochrome c oxidase. Methemoglobin binds cyanide to form **cyanomethemoglobin**, thereby reducing the amount of free cyanide available to inhibit cellular respiration. This action effectively mitigates the toxic effects of cyanide on cellular respiration.

## Why Each Wrong Option is Incorrect
- **Option A:** This option might suggest a mechanism not directly related to methemoglobin formation or direct binding of cyanide. Without specifics, it's hard to refute directly, but given the context, it's less likely to involve direct chelation or other mechanisms not primarily related to methemoglobin.
- **Option B:** This could potentially involve another pathway or mechanism not directly linked to the formation of methemoglobin or the direct detoxification of cyanide through binding.
- **Option C:** Similar to Option A, without specifics, it's challenging to address directly, but the focus on methemoglobin formation is key in cyanide poisoning treatment.

## Clinical Pearl / High-Yield Fact
A critical clinical pearl is that **hydroxocobalamin (vitamin B12a)** can also be used to treat cyanide poisoning by directly binding cyanide, forming cyanocobalamin (vitamin B12), which is excreted by the kidneys. However, the question specifically asks about sodium nitroprusside's mechanism, which revolves around **methemoglobin** formation.

## Correct Answer: D. Formation of methemoglobin.