**Question:** Which of the following is not a free radical, but it is a reactive oxygen species?

A. Superoxide (O₂•-)
B. Hydrogen peroxide (H₂O₂)
C. Nitric oxide (NO)
D. Lipid hydroperoxides (ROOH)

**Core Concept:** Reactive oxygen species (ROS) are molecules that contain unpaired electrons, leading to their reactivity and potential for causing damage to cellular components. Free radicals are highly reactive molecules with an unpaired electron in their outer shell, which makes them unstable and highly reactive.

**Why the Correct Answer is Right:** Hydrogen peroxide (H₂O₂) is a reactive oxygen species but is not a free radical. It is produced during cellular respiration, and it can be converted into highly reactive hydroxyl radicals (OH•) and hydrogen peroxide (H₂O₂) when reacting with the enzyme catalase and hydrogen peroxide.

**Why Each Wrong Option is Incorrect:**

A. Superoxide (O₂•-) is a free radical due to its unpaired electron in the outer shell, which makes it highly reactive and potentially damaging to cellular components.
B. Nitric oxide (NO) is also a free radical, as it has an unpaired electron in the outer shell and is reactive.
C. Nitric oxide (NO) is a reactive nitrogen species, not a reactive oxygen species. Although it can react with superoxide (O₂•-) to form highly reactive peroxynitrite anion (ONOO-), this is not considered as a correct answer choice.
D. Lipid hydroperoxides (ROOH) are reactive oxygen species, but they are not free radicals. Free radicals are highly reactive due to their unpaired electrons, while lipid hydroperoxides are reactive due to their reactive oxygen species nature but lack the characteristic unpaired electron in the outer shell.

**Clinical Pearl:** Reactive oxygen species play a crucial role in cellular signaling, immune response, and defense against pathogens. However, they can cause oxidative stress and damage cellular components when produced in excess, contributing to various diseases, including neurodegenerative disorders, cardiovascular diseases, and cancer.