**Core Concept**
Cortisol is a steroid hormone produced by the adrenal cortex that plays a crucial role in the body's response to stress, regulating blood sugar levels, and maintaining immune function. Cortisol exerts its effects by binding to glucocorticoid receptors, which are present in various tissues throughout the body.

**Why the Correct Answer is Right**
Cortisol increases the expression of genes that promote the breakdown of muscle tissue and the release of glucose from stored energy sources. This is achieved through the activation of the glucocorticoid receptor, which then translocates to the nucleus and binds to specific DNA sequences, leading to the transcription of genes involved in gluconeogenesis and lipolysis. Cortisol also inhibits the release of insulin and promotes the release of glucagon, further contributing to the increase in blood glucose levels.

**Why Each Wrong Option is Incorrect**
**Option A:** Glycogen synthesis - Cortisol actually inhibits glycogen synthesis by suppressing the activity of glycogen synthase.
**Option B:** Lipid synthesis - Cortisol promotes the breakdown of lipids, increasing the availability of fatty acids for energy production.
**Option C:** Protein synthesis - Cortisol actually inhibits protein synthesis by suppressing the activity of ribosomal RNA and reducing the translation of mRNA into protein.

**Clinical Pearl / High-Yield Fact**
Cortisol has a negative feedback effect on the hypothalamic-pituitary-adrenal (HPA) axis, which is a critical regulatory mechanism that maintains homeostasis in response to stress.

**Correct Answer: A. Glycogen synthesis**