Which of the following is the most specific antidote for cyanide poisoning that directly binds and inactivates cyanide ions?

Explanation

## Antidotes for Cyanide Poisoning **Key Point:** Hydroxocobalamin is the most specific and preferred antidote because it directly binds cyanide ions, forming non-toxic cyanocobalamin, which is then excreted in urine. ### Comparison of Antidotes | Antidote | Mechanism | Advantage | Disadvantage | |----------|-----------|-----------|---------------| | **Hydroxocobalamin** | Direct binding to CN⁻ → cyanocobalamin | Highly specific, rapid, safe, no methemoglobinemia risk | Expensive, large volume IV required | | **Sodium nitrite** | Induces methemoglobinemia to bind CN⁻ | Fast onset, available | Risk of methemoglobinemia toxicity, less specific | | **Sodium thiosulfate** | Provides sulfur for rhodanese enzyme to convert CN⁻ → thiocyanate | Synergistic with nitrite, safe | Slower acting, requires rhodanese enzyme | | **Methylene blue** | Electron acceptor in anaerobic metabolism | Treats lactic acidosis indirectly | Not a direct cyanide antidote, not first-line | ### Treatment Algorithm ```mermaid flowchart TD A[Suspected Cyanide Poisoning]:::outcome --> B{Severity?}:::decision B -->|Severe/Symptomatic| C[Hydroxocobalamin IV 5g]:::action B -->|Mild/Asymptomatic| D[Supportive care + monitoring]:::action C --> E[Repeat dose if needed]:::action E --> F[Urine monitoring for thiocyanate]:::action D --> G[Consider hydroxocobalamin if worsens]:::action ``` **High-Yield:** Hydroxocobalamin is now the **first-line antidote** in most developed countries (FDA approved 2006) because it: - Does not cause methemoglobinemia - Has a wider safety margin - Works faster than the traditional nitrite-thiosulfate combination - Is effective even at high cyanide doses **Mnemonic:** **HITS** = Hydroxocobalamin, Immediate, Targets cyanide, Specific **Clinical Pearl:** In resource-limited settings where hydroxocobalamin is unavailable, the traditional **cyanide antidote kit** (sodium nitrite + sodium thiosulfate) remains the standard. Sodium nitrite is given first to induce methemoglobinemia (which binds CN⁻), followed by thiosulfate to enhance conversion to thiocyanate.