**Core Concept**
Cytochrome C oxidase (Complex IV) is a crucial enzyme in the mitochondrial electron transport chain, responsible for transferring electrons from cytochrome C to oxygen, generating a proton gradient for ATP synthesis. Its inhibition disrupts the electron transport chain, leading to impaired energy production and potentially cell death.

**Why the Correct Answer is Right**
The correct answer is cyanide. Cyanide is a potent inhibitor of cytochrome C oxidase, binding to the iron atom at the active site and preventing the transfer of electrons to oxygen. This inhibition is highly specific and irreversible, leading to rapid cell death. The high affinity of cyanide for cytochrome C oxidase makes it an effective poison, even at low concentrations.

**Why Each Wrong Option is Incorrect**
**Option A:** Rotenone is an inhibitor of Complex I (NADH dehydrogenase) in the electron transport chain, not cytochrome C oxidase. It blocks the transfer of electrons from NADH to ubiquinone, but does not affect cytochrome C oxidase.

**Option B:** Antimycin A is an inhibitor of Complex III (cytochrome b-c1 complex) in the electron transport chain. It blocks the transfer of electrons from cytochrome b to cytochrome c1, but does not affect cytochrome C oxidase.

**Option C:** Oligomycin is an inhibitor of Complex V (ATP synthase) in the electron transport chain. It blocks the production of ATP from the proton gradient, but does not affect cytochrome C oxidase.

**Clinical Pearl / High-Yield Fact**
Cyanide poisoning is a medical emergency that requires immediate treatment. The antidote for cyanide poisoning is hydroxocobalamin, which binds to cyanide and forms a nontoxic complex that can be excreted by the kidneys. Early recognition and treatment of cyanide poisoning can save lives.

**Correct Answer: D. Cyanide**