**Question:** Selenium is a cofactor in the following enzyme:

A. Glutathione peroxidase
B. Deiodinase
C. Thyroxine-binding globulin
D. Tyrosine ammonia lyase

**Core Concept:** Selenium is an essential trace element that plays a crucial role in human health. It is part of many selenoproteins, particularly glutathione peroxidase, which is involved in antioxidant defense and protection against oxidative stress.

**Why the Correct Answer is Right:** Selenium is a key component in the enzyme Glutathione peroxidase (GPx), which is responsible for catalyzing the reduction of hydrogen peroxide and organic hydroperoxides to water and alcohol, respectively. This process helps protect cells from oxidative damage.

**Why Each Wrong Option is Incorrect:**

A. Deiodinase is an enzyme involved in thyroid hormone synthesis and action, not related to selenium.
B. Tyrosine ammonia lyase is an enzyme involved in the breakdown of amino acids, unrelated to selenium.
C. Thyroxine-binding globulin is a transport protein for thyroid hormones, not an enzyme related to selenium.

**Clinical Pearl:** Although all options except B contain the word "enzyme," the correct answer, A, is the only one related to selenium and its biological role as a cofactor in antioxidant defense.

**Correct Answer:** A. Glutathione peroxidase

**Explanation:** Selenium functions as a cofactor in various enzymes, but only option A is directly related to selenium's role in antioxidant defense and protection against oxidative stress. The other options involve unrelated processes or enzymes.