## Core Concept
Cyanide poisoning acts by inhibiting a crucial enzyme in the cellular respiration pathway. Cellular respiration is a process in which cells break down glucose to produce energy, which is then stored in the form of ATP. This process primarily occurs in the mitochondria and involves several key enzymes and electron transport chains.

## Why the Correct Answer is Right
The correct answer, **C. Cytochrome c oxidase**, is the enzyme inhibited by cyanide. Cytochrome c oxidase, also known as Complex IV, is the final enzyme in the electron transport chain of mitochondria. It plays a critical role in transferring electrons to oxygen, which is the final electron acceptor, resulting in the formation of water. This process is essential for generating ATP during oxidative phosphorylation. Cyanide binds to the iron-copper binuclear center of cytochrome c oxidase, thereby blocking the transfer of electrons to oxygen. This inhibition halts the electron transport chain and oxidative phosphorylation, leading to cellular asphyxiation and death, as cells can no longer produce ATP through aerobic respiration.

## Why Each Wrong Option is Incorrect
- **Option A: Phosphofructokinase** - This enzyme is involved in glycolysis, the process of breaking down glucose to pyruvate. While inhibition of phosphofructokinase would affect energy production, it is not the direct target of cyanide.
- **Option B: Succinate dehydrogenase** - Also known as Complex II or succinate:quinone oxidoreductase, this enzyme participates in both the citric acid cycle and the electron transport chain. However, it is not the site of action for cyanide.
- **Option D: Pyruvate kinase** - This enzyme is involved in the final step of glycolysis, converting phosphoenolpyruvate (PEP) into pyruvate. It is not directly related to the mechanism of cyanide poisoning.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that the treatment for cyanide poisoning includes the use of **hydroxocobalamin** (Vitamin B12a) or **sodium nitrite** and **sodium thiosulfate**. Hydroxocobalamin works by binding to cyanide to form cyanocobalamin (Vitamin B12), which is then excreted. Sodium nitrite and sodium thiosulfate work by generating methemoglobin, which has a high affinity for cyanide, and by providing a sulfur donor for the enzyme rhodanese to convert cyanide into thiocyanate, respectively.

## Correct Answer: C. Cytochrome c oxidase