**Core Concept**
Steroids are a class of lipid molecules with a specific carbon skeleton and four fused rings. They have a wide range of biological functions, including hormone regulation, immune responses, and cell membrane structure. In pharmacology, steroids are often used to mimic or modulate the body's natural steroid hormones.

**Why the Correct Answer is Right**
The correct answer is a molecule that does not belong to the steroid class. Steroids typically have a planar, ring-shaped structure and are characterized by the presence of a hydroxyl group (-OH) at one of the carbon atoms. They can be further classified into different types, such as corticosteroids, anabolic steroids, and sex steroids, based on their biological function.

**Why Each Wrong Option is Incorrect**
**Option A:** Cortisol is a corticosteroid hormone produced by the adrenal gland, which is a type of steroid.
**Option B:** Testosterone is a sex steroid hormone produced by the testes in males, which is also a type of steroid.
**Option C:** Dexamethasone is a synthetic corticosteroid medication used to treat various inflammatory and autoimmune disorders, which is a type of steroid.

**Clinical Pearl / High-Yield Fact**
It's essential to note that steroids are not the same as prostaglandins, which are a different class of lipid molecules that play a crucial role in inflammation and immune responses. The distinction between steroids and prostaglandins is critical in pharmacology, as they have different mechanisms of action and are used to treat different conditions.

**Correct Answer:** C.