A 35-year-old industrial worker is brought to the emergency department with altered mental status, seizures, and profound hypotension 30 minutes after exposure to hydrogen cyanide gas in a chemical plant accident. Arterial blood gas shows normal oxygen saturation (98%) and adequate oxygen delivery, yet the patient is in profound shock with lactic acidosis (pH 7.18, lactate 8.2 mmol/L). The structure marked **C** in the electron transport chain diagram is the site of cyanide's lethal action. Which of the following best describes the mechanism of cyanide toxicity in this clinical context?

Explanation

## Why option 1 is correct Cyanide's lethal mechanism is direct inhibition of Complex IV (cytochrome c oxidase), the terminal enzyme of the electron transport chain marked as **C**. Cyanide binds with extremely high affinity to the ferric iron (Fe³⁺) in heme a₃ of cytochrome c oxidase, forming a stable CN⁻–Fe³⁺ complex. This prevents the final electron transfer to molecular oxygen, halting the entire ETC. Critically, oxygen remains available in the blood and tissues (hence normal SpO₂), but cells cannot use it for ATP production — this is the definition of histotoxic hypoxia. The resulting energy crisis causes cellular dysfunction, lactic acidosis from anaerobic metabolism, and rapid cardiovascular collapse. (Harper 32e Ch 13; KD Tripathi 9e poisoning section) ## Why each distractor is wrong - **Option 2**: Complex I inhibition would slow the ETC but would not produce the acute, complete blockade seen with cyanide. Cyanide does not primarily target Complex I; its action is specific to Complex IV. Students may confuse this with other ETC poisons that do affect earlier complexes. - **Option 3**: Uncoupling agents (like DNP or aspirin overdose) dissipate the proton gradient and prevent ATP synthesis, but oxygen consumption actually *increases* in uncoupling because the ETC runs faster. Cyanide causes the opposite — complete ETC arrest. The clinical presentation (shock, lactic acidosis) reflects energy failure, not uncoupling. - **Option 4**: Cyanide does not inhibit Complex III or interfere with cytochrome c binding. Complex III dysfunction would not produce the rapid, irreversible toxicity seen with cyanide. This is a plausible distractor for students who confuse the site of action. **High-Yield:** Cyanide = histotoxic hypoxia (normal O₂ delivery, zero O₂ utilization); antidote = hydroxocobalamin or nitrite + thiosulfate to form methemoglobin that binds cyanide. [cite: Harper 32e Ch 13; KD Tripathi 9e poisoning section]