**Question:** Which of the following is not steroid?

A. Cortisol
B. Testosterone
C. Cholesterol
D. Estrogen

**Core Concept:** Steroids are a group of lipid compounds that share some structural and functional similarities. They are synthesized from cholesterol in the endoplasmic reticulum of cells. Steroids play a crucial role in various physiological processes, including hormone secretion, immune response, and cell signaling.

**Why the Correct Answer is Right:** Cholesterol is the precursor molecule for steroid hormone synthesis. While it is essential for the production of steroid hormones, cholesterol itself is not a steroid hormone and does not possess biological activity.

**Why Each Wrong Option is Incorrect:**

A. Cortisol: A potent glucocorticoid hormone, involved in the body's response to stress and immune system regulation.
B. Testosterone: An androgen hormone, essential for male sex characteristics and reproductive functions.
D. Estrogen: A group of steroid hormones responsible for female sex characteristics and reproductive functions.

**Why Cholesterol is Not a Steroid Hormone:** Cholesterol is involved in steroid hormone synthesis but lacks the characteristic 17α-hydroxyl group and 11β-hydroxyl group found in steroid hormones. Steroid hormones are biologically active, while cholesterol is an inactive precursor.

**Clinical Pearl:** Understanding the role of cholesterol in steroid hormone synthesis is essential in understanding endocrinology and hormone regulation in the body. Adequate cholesterol levels are crucial for maintaining normal hormone levels and overall health.

**Correct Answer:** **Cholesterol (Option C)** is not a steroid hormone as it is the precursor molecule for steroid hormone synthesis and lacks the characteristic functional groups found in steroid hormones.