**Question:** Least toxic organophosphorus compound is:

A. Diisopropylfluorophosphate (DFP)
B. Malathion
C. Parathion
D. Fenitrothion

**Core Concept:** Organophosphorus compounds (OPs) are a class of chemicals that inhibit acetylcholinesterase (AChE), an enzyme responsible for breaking down the neurotransmitter acetylcholine, leading to overactivity of the neuromuscular junction and other organs. This results in symptoms like muscle weakness, respiratory distress, and ultimately death.

**Why the Correct Answer is Right:**

Diisopropylfluorophosphate (DFP) is considered the least toxic among the given options due to its high lipophilicity, allowing it to easily cross cell membranes and bind to AChE. This binding leads to a slower release of acetylcholine and a lower risk of respiratory depression and other toxic effects.

**Why Each Wrong Option is Incorrect:**

B. Malathion: A lipophilic OP, it can bind to AChE, leading to toxicity.
C. Parathion: A lipophilic OP, it can bind to AChE, causing toxicity.
D. Fenitrothion: A lipophilic OP, it can bind to AChE, causing toxicity.

**Why the Correct Answer is Right:** Organophosphorus compounds act as AChE inhibitors, leading to toxic effects. DFP stands out as the least toxic due to its high lipophilicity, which slows down the release of acetylcholine and reduces toxicity.

**Why Each Wrong Option is Incorrect:** Malathion, Parathion, and Fenitrothion are lipophilic OP compounds, making them capable of binding to AChE and causing toxicity.

**Clinical Pearl:** Acetylcholinesterase inhibitors are used in warfare and suicide poisoning, with antidotes like atropine and pralidoxime chloride used for treatment. Understanding the toxicity of OP compounds and their mechanism of action is crucial for clinical practice, especially in toxicology and emergency medicine.