## **Core Concept**
The antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating water balance in the body. It acts primarily on the kidneys to increase water reabsorption. ADH is released by the posterior pituitary gland and is synthesized in the hypothalamus.

## **Why the Correct Answer is Right**
ADH increases water reabsorption in the collecting ducts of the nephrons in the kidneys. This action is mediated through the **V2 receptors** on the basolateral side of the collecting duct cells. When ADH binds to these receptors, it triggers a signaling cascade that results in the insertion of **aquaporin-2 water channels** into the apical membrane of the collecting duct cells, thereby increasing water permeability and reabsorption. This process concentrates the urine and reduces water loss from the body.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because ADH does not decrease water reabsorption; it does the opposite.
- **Option B:** This option is incorrect as ADH does not primarily act by increasing sodium reabsorption, although it can have some effect on sodium handling indirectly; its primary action related to water balance is through increasing water reabsorption.
- **Option C:** This option might seem plausible but is not accurate regarding the primary action of ADH. ADH does not significantly increase glomerular filtration rate (GFR) as its primary mechanism; its main effect is on water reabsorption in the collecting ducts.

## **Clinical Pearl / High-Yield Fact**
A key clinical correlation is that **diabetes insipidus**, a condition characterized by the deficiency of ADH (central diabetes insipidus) or resistance to ADH (nephrogenic diabetes insipidus), leads to the production of large volumes of dilute urine, resulting in polyuria and polydipsia. This condition highlights the critical role of ADH in maintaining water balance.

## **Correct Answer:** . Increased water reabsorption in collecting duct.