Which of the following is considered a reactive oxygen species (free radical)?

The hydroxyl radical is considered a reactive oxygen species (ROS) and is one of the most reactive free radicals formed in biological systems. It is generated through various metabolic processes and can lead to oxidative stress, damaging cellular components such as DNA, proteins, and lipids. The hydroxyl radical plays a significant role in numerous physiological and pathological processes, including inflammation and cellular signaling.

In contrast, ozone is a triatomic molecule that can act as a reactive species but is more accurately classified as an oxidant rather than a free radical in the context of ROS. Carbon dioxide is a stable molecule that does not exhibit free radical characteristics and does not contribute to oxidative stress. Similarly, hydrogen sulfide is a gaseous signaling molecule but does not fall under the category of reactive oxygen species. Understanding these distinctions is critical for recognizing how different species interact with biological systems, particularly in the context of oxidative stress and related conditions.