**Question:** Aldosterone is secreted by-

A. Adrenal medulla
B. Adrenal cortex
C. Pituitary gland
D. Thymus gland

**Core Concept:** Aldosterone is a hormone produced primarily by the zona glomerulosa of the adrenal cortex, in response to low blood sodium levels and high blood potassium levels or low blood potassium levels. It plays a crucial role in regulating electrolyte balance and fluid volume in the body.

**Why the Correct Answer is Right:** The correct answer, B, explains that aldosterone is secreted by the adrenal cortex, particularly the zona glomerulosa. This response helps maintain electrolyte balance, as aldosterone acts on the kidneys to increase sodium retention, which leads to water retention and increased blood volume (hypertension). Aldosterone also promotes potassium excretion to counteract its retention, ensuring a stable potassium level in the blood.

**Why Each Wrong Option is Incorrect:**

A. Aldosterone is not secreted by the adrenal medulla, which is responsible for producing catecholamines like adrenaline and noradrenaline.

C. Aldosterone is not secreted by the pituitary gland, which is responsible for producing hormones like growth hormone, prolactin, and adrenocorticotropic hormone (ACTH). ACTH stimulates aldosterone secretion indirectly, but the hormone itself is produced by the pituitary.

D. Aldosterone is not secreted by the thymus gland, which is responsible for immune system development and T-cell maturation. The thymus gland is not involved in regulating electrolyte balance or hormone secretion.

**Clinical Pearl:** Understanding the role of aldosterone in regulating electrolyte balance aids in diagnosing and managing conditions involving aldosterone imbalances, such as primary aldosteronism (Conn's syndrome) and Addison's disease. Recognizing the source of aldosterone (adrenal cortex) helps differentiate between various disorders and tailor appropriate treatment strategies.