**Core Concept**
Adrenaline, also known as epinephrine, is a catecholamine hormone and neurotransmitter that plays a crucial role in the body's "fight or flight" response. It is secreted by the adrenal glands in response to stress, fear, or anxiety.

**Why the Correct Answer is Right**
Adrenaline is secreted by the adrenal medulla, a part of the adrenal glands located above the kidney. The adrenal medulla contains chromaffin cells that synthesize and store adrenaline. When stimulated by the hypothalamus-pituitary-adrenal axis, these cells release adrenaline into the bloodstream, leading to increased heart rate, blood pressure, and energy mobilization. This response is mediated by the beta-adrenergic receptors in the heart, lungs, and other organs.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because adrenaline is not secreted by the adrenal cortex, which produces steroid hormones like cortisol and aldosterone.
**Option B:** This option is incorrect because the pancreas secretes insulin and glucagon, not adrenaline.
**Option C:** This option is incorrect because the thyroid gland produces thyroid hormones, not adrenaline.

**Clinical Pearl / High-Yield Fact**
The adrenal glands are a common source of Cushing's syndrome, a condition characterized by excess cortisol production. However, excess adrenaline production is not typically associated with a specific syndrome, but rather with conditions like pheochromocytoma, a rare tumor of the adrenal medulla.

**Correct Answer: C. The adrenal medulla.**