Mechanism of cyanide poisoning is by inhibiting –
**Core Concept**
Cyanide poisoning is a chemical asphyxiant that disrupts cellular respiration by inhibiting the electron transport chain in mitochondria. This leads to a rapid depletion of ATP, causing cellular dysfunction and ultimately death.
**Why the Correct Answer is Right**
Cyanide binds to the iron atom in cytochrome c oxidase (Complex IV) of the electron transport chain, preventing the transfer of electrons and thus halting oxidative phosphorylation. This results in a significant decrease in ATP production, leading to cellular energy depletion and death. The affinity of cytochrome c oxidase for oxygen is also reduced, causing a shift towards anaerobic metabolism and further exacerbating the energy crisis.
**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because cyanide does not directly inhibit the citric acid cycle or the electron transport chain at Complex I. While Complex I is also an important site for electron transfer, cyanide's primary action is at Complex IV.
**Option B:** This is incorrect because cyanide does not directly inhibit the Na+/K+ ATPase pump or other ion channels. Its primary mechanism of action is related to energy production, not ion balance.
**Option C:** This is incorrect because cyanide does not directly inhibit the synthesis of ATP from ADP and Pi. Its primary mechanism of action is related to the electron transport chain, not substrate-level phosphorylation.
**Clinical Pearl / High-Yield Fact**
Cyanide poisoning can be treated with hydroxocobalamin, which binds to cyanide to form cyanocobalamin, a less toxic compound that can be excreted by the kidneys. This is a classic example of a "chelation therapy" used to counteract the effects of a toxic substance.
**Correct Answer: C. Cytochrome c oxidase (Complex IV) of the electron transport chain.**