**Core Concept**
Superoxide dismutase is an enzyme that 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 the brain against oxidative damage caused by free radicals.

**Why the Correct Answer is Right**
Superoxide dismutase (SOD) is a key antioxidant enzyme found in nearly all living cells. It is particularly abundant in the brain, where it helps to neutralize superoxide anions generated during normal metabolic processes. By converting superoxide into hydrogen peroxide and oxygen, SOD prevents the formation of more reactive and damaging free radicals, such as hydroxyl radicals (·OH). This enzyme is essential for maintaining the integrity of brain cells and protecting against neurodegenerative diseases.

**Why Each Wrong Option is Incorrect**
**Option A:** Myeloperoxidase is an enzyme found in neutrophils that produces hypochlorous acid to kill ingested microorganisms, but it does not play a role in protecting the brain from free radical injury.
**Option C:** Monoamine oxidase (MAO) is an enzyme involved in the breakdown of neurotransmitters such as dopamine, serotonin, and norepinephrine, but it does not have antioxidant properties.
**Option D:** Hydroxylase enzymes are involved in various biochemical reactions, including the synthesis of steroids and the degradation of amino acids, but they do not protect the brain from free radical injury.

**Clinical Pearl / High-Yield Fact**
Superoxide dismutase is a crucial enzyme for maintaining cellular health, particularly in the brain. A deficiency in SOD has been implicated in various neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease.

**✓ Correct Answer: B. Superoxide dismutase**