AAMC FL Practice Exam 2025 – The Comprehensive All-in-One Guide to Exam Success!

The hormone that decreases water excretion in the body is antidiuretic hormone (ADH), also known as vasopressin. ADH plays a critical role in regulating the body's water balance. When the body is dehydrated or when blood osmolarity increases, ADH is released from the posterior pituitary gland. The primary action of ADH is to increase the permeability of the collecting ducts in the kidneys, allowing more water to be reabsorbed back into the bloodstream rather than being excreted in the urine. This process helps to concentrate the urine and conserve water, thus reducing water excretion.

Other hormones mentioned in the options serve different roles. Aldosterone primarily regulates sodium and potassium balance and, to a lesser extent, affects water retention indirectly through sodium reabsorption. Atrial natriuretic peptide (ANP) promotes natriuresis, leading to increased sodium and water excretion, thus opposing the action of ADH. Cortisol, a glucocorticoid hormone, influences metabolism and has some effect on water balance but does not directly decrease water excretion as ADH does.

Therefore, ADH is the hormone that specifically decreases water excretion by enhancing water reabsorption in the kidneys.