## **Core Concept**
ADH, or Antidiuretic Hormone, also known as vasopressin, plays a crucial role in regulating water balance in the body. It acts primarily on the kidneys to influence water reabsorption. The hormone is produced by the hypothalamus and released by the posterior pituitary gland.

## **Why the Correct Answer is Right**
The correct answer, collecting ducts, is where ADH exerts its primary effect. ADH acts on the collecting ducts of the nephrons in the kidneys by binding to **V2 receptors**, which are specifically located on the basolateral surface of the collecting duct cells. This binding leads to the insertion of **aquaporin-2** water channels into the apical membrane of these cells, increasing the permeability of the collecting ducts to water. As a result, more water is reabsorbed into the bloodstream, concentrating the urine and reducing its volume. This mechanism is crucial for maintaining proper water balance and osmolarity of the blood.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Proximal convoluted tubule. While a significant amount of water reabsorption occurs here, it is not primarily under the control of ADH. The reabsorption in the proximal convoluted tubule is largely isotonic and is driven by the reabsorption of solutes like sodium and glucose.
- **Option B:** Descending limb of loop of Henle. This part of the nephron is highly permeable to water due to the presence of aquaporin-1 water channels, but its permeability is not significantly influenced by ADH. The water reabsorption here is a passive process, driven by the osmotic gradient in the medulla.
- **Option C:** Ascending limb of loop of Henle. This segment is impermeable to water but actively reabsorbs ions like sodium, potassium, and chloride. The reabsorption here contributes to the formation of the osmotic gradient that allows for water reabsorption in other parts of the nephron.

## **Clinical Pearl / High-Yield Fact**
A key clinical correlation is that in **central diabetes insipidus**, a deficiency of ADH leads to an inability to reabsorb water in the collecting ducts, resulting in the production of large volumes of dilute urine. This condition can be treated with **desmopressin**, a synthetic analogue of ADH.

## **Correct Answer:** . Collecting ducts.