An enzyme that protects the brain from free radical injury is
**Core Concept**
The question is testing the student's knowledge of the body's defense mechanisms against oxidative stress and free radical injury, particularly in the context of brain protection.
**Why the Correct Answer is Right**
The enzyme that protects the brain from free radical injury is Superoxide Dismutase (SOD). SOD catalyzes the dismutation of the superoxide (O2−) radical into either ordinary molecular oxygen (O2) or hydrogen peroxide (H2O2). This enzyme plays a critical role in defending cells against the toxic effects of oxygen radicals, which are formed during normal aerobic metabolism. In the brain, SOD is present in high concentrations and is involved in the defense against excitotoxicity and neurodegenerative diseases.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the protection of the brain from free radical injury.
**Option B:** While this enzyme is involved in antioxidant defense, it is not the primary enzyme responsible for protecting the brain from free radical injury.
**Option C:** This option is not directly related to the protection of the brain from free radical injury.
**Clinical Pearl / High-Yield Fact**
Superoxide Dismutase (SOD) is a key enzyme in the body's antioxidant defense system, and its activity is essential for maintaining the balance between oxidative stress and antioxidant defenses in the brain.
**Correct Answer:** D. Superoxide Dismutase (SOD)