**Question:** Cyanide inhibits

A. Cytochrome c oxidase
B. Sodium channels
C. Nitric oxide synthase
D. Acetylcholine receptors

**Core Concept:** Cyanide is a toxic compound that inhibits cellular respiration by blocking the electron transport chain in mitochondria. Cytochrome c oxidase (complex IV) is a crucial enzyme involved in this process.

**Why the Correct Answer is Right:** Cyanide binds to the iron-sulfur centers of cytochrome c oxidase, preventing the binding of oxygen and ultimately blocking the electron transport chain. This leads to a decrease in ATP production and cellular dysfunction.

**Why Each Wrong Option is Incorrect:**

B. Sodium channels: Cyanide does not interfere with sodium channels. Sodium channels are involved in nerve and muscle function but are unrelated to cellular respiration.

C. Nitric oxide synthase: Nitric oxide synthase is involved in nitric oxide production, which plays a role in vascular dilation and neurotransmission, but it is not related to cellular respiration or cyanide inhibition.

D. Acetylcholine receptors: Acetylcholine receptors are involved in neuromuscular transmission, but they are not related to the inhibition of cellular respiration or cyanide's effect on cytochrome c oxidase.

**Core Concept:** Cyanide's effect on cytochrome c oxidase is a key mechanism in understanding its toxic effects on cellular respiration and overall cellular function.

**Clinical Pearl:** Cyanide poisoning is a life-threatening emergency, requiring prompt treatment with hydroxocobalamin (Cyanokit) and/or sodium thiosulfate. Early recognition and treatment are essential to prevent neurological and cardiovascular complications.