What is the mechanism of aldosterone?
a. it causes sodium reabsorption and potassium excretion.
b. it activates angiotensin.
c. it stimulates the secretion of renin.
d. it stimulates the secretion of water

The correct answer is A.

Explanation:

Aldosterone is a steroid hormone, produced by the adrenal cortex, in the adrenal gland.  

Aldosterone is part of the renin-angiotensin-aldosterone system which helps to maintain blood pressure.

When renal blood flow drops, juxtaglomerular cells in the kidney convert prorenin into renin. Renin then converts angiotensinogen (released by the liver) into angiotensin I, that then is converted to angiotensin II by angiotensin converting enzyme in the lung.

Angiotensin II is a potent vasoconstrictive peptide that helps to narrow vessels and also stimulates aldosterone secretion.

At this point, aldosterone increases sodium reabsorption and potassium excretion (to maintain electrolyte balance).

The final result is an increase in extracellular fluid volume and therefore in blood pressure.