## **Core Concept**
The question tests the understanding of antioxidant defense mechanisms, specifically focusing on enzymes that act as free radical scavengers. Free radicals are unstable molecules that can cause oxidative stress, leading to cell damage. The body has developed antioxidant defenses, including enzymes, to neutralize these harmful radicals.

## **Why the Correct Answer is Right**
Superoxide dismutase (SOD) is a key enzyme that catalyzes the dismutation of the superoxide (O2−) radical into either ordinary molecular oxygen (O2) or hydrogen peroxide (H2O2). This reaction is crucial because superoxide is a reactive oxygen species that can cause cellular damage. By converting superoxide into less harmful substances, SOD acts as a free radical scavenger, protecting cells from oxidative stress.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While Glutathione peroxidase is indeed an antioxidant enzyme, its primary role is to reduce hydrogen peroxide to water and lipid hydroperoxides to their corresponding alcohols, using glutathione as a reducing agent. It does not directly scavenge free radicals like superoxide.
- **Option B:** Catalase is another antioxidant enzyme that decomposes hydrogen peroxide into water and oxygen. It does not directly act on superoxide radicals.
- **Option D:** NADPH is a coenzyme involved in anabolic reactions, such as lipid and nucleic acid synthesis, and in antioxidant defense mechanisms, like maintaining glutathione in its reduced form. However, it is not an enzyme and does not directly scavenge free radicals.

## **Clinical Pearl / High-Yield Fact**
A crucial point to remember is that **Superoxide dismutase (SOD)** comes in different isoforms, including SOD1 (cytosolic), SOD2 (mitochondrial), and SOD3 (extracellular). Deficiencies or mutations in SOD1 have been associated with familial amyotrophic lateral sclerosis (ALS), highlighting the enzyme's importance in neuroprotection.

## **Correct Answer:** C. Superoxide dismutase.