**Question:** Mechanism of action of cyanide poison is

A. Inhibition of cytochrome oxidase
B. Inhibition of ATP synthesis
C. Interaction with RNA synthesis
D. Inhibition of Na+/K+-ATPase

**Core Concept:** Cyanide is a highly toxic substance that can interfere with cellular respiration, leading to severe health complications and potentially death. Its primary mechanism of action involves inhibiting the enzyme cytochrome oxidase, which is crucial for aerobic respiration in mitochondria.

**Why the Correct Answer is Right:** Cyanide poisoning primarily targets the mitochondria, where it binds to the heme group of cytochrome oxidase, an essential enzyme involved in the electron transport chain during cellular respiration. This binding prevents the binding of oxygen to cytochrome oxidase, ultimately leading to the inhibition of cellular respiration and ATP synthesis, and causing a cascade of detrimental effects in the body.

**Why Each Wrong Option is Incorrect:**

A. Inhibition of cytochrome oxidase is the primary mechanism of cyanide poisoning, not inhibition of ATP synthesis.
B. While cyanide poisoning can disrupt ATP synthesis, the answer provided is incorrect because cytochrome oxidase inhibition is the primary mechanism, not ATP synthesis inhibition.
C. Cyanide poisoning does not primarily involve interaction with RNA synthesis; the correct mechanism is cytochrome oxidase inhibition.
D. Inhibition of Na+/K+-ATPase is unrelated to cyanide poisoning, as the primary mechanism involves cytochrome oxidase inhibition, not this enzyme.

**Clinical Pearl:** Understanding the mechanism of cyanide poisoning is essential for clinical practice, as prompt treatment with sodium thiosulfate can counteract cyanide toxicity by reacting with cyanide to form nontoxic compounds. This knowledge is vital for treating patients exposed to cyanide poisoning, potentially saving lives in emergency situations.