**Question:** Free radicals are chemical species which have

A. Unpaired electrons
B. High reactivity
C. Role in cell signalling
D. Low reactivity

**Correct Answer:** A. Unpaired electrons

**Core Concept:** Free radicals are reactive oxygen or nitrogen molecules containing one or more unpaired electrons in their outer shell. They are highly reactive due to their high electronegativity and tendency to react with other molecules to stabilize their unpaired electron configuration.

**Why the Correct Answer is Right:** The correct answer, A. Unpaired electrons, is right because free radicals possess unpaired electrons in their outer shells, which makes them highly reactive due to their high electronegativity. This reactivity leads them to react with other molecules to stabilize their electron configuration, often causing damage to cellular components and contributing to oxidative stress.

**Why Each Wrong Option is Incorrect:**

B. High reactivity (Option B) is partially correct but incomplete. While free radicals are indeed highly reactive, the core concept of unpaired electrons is essential to understanding their properties and behavior.
C. Role in cell signalling (Option C) is a related concept but not the defining characteristic of free radicals. Free radicals can play a role in cell signalling, but their primary feature is the presence of unpaired electrons.
D. Low reactivity (Option D) is incorrect because free radicals are highly reactive due to their unpaired electrons, not low reactivity.

**Clinical Pearl:** Unpaired electrons in free radicals are crucial for understanding their reactivity and role in cellular processes, such as oxidative stress and inflammation. Knowledge of this concept is essential for medical professionals to effectively diagnose and treat conditions involving free radicals and oxidative stress.