## **Core Concept**
The Antidiuretic Hormone (ADH), also known as **vasopressin**, plays a crucial role in regulating water balance in the body. It is secreted by the posterior pituitary gland and acts primarily on the kidneys. ADH helps to control the amount of water reabsorbed by the kidneys, thereby influencing urine concentration and volume.

## **Why the Correct Answer is Right**
The correct answer, **D.**, highlights that ADH primarily promotes **water reabsorption in the collecting ducts of the nephrons** in the kidneys. This action is mediated through **V2 receptors** on the basolateral side of the collecting duct cells, stimulating the insertion of **aquaporin-2 water channels** into the apical membrane. This increases the permeability of the collecting ducts to water, allowing more water to be reabsorbed into the bloodstream, which concentrates the urine and reduces its volume.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because ADH does not primarily act by increasing sodium reabsorption in the proximal convoluted tubule; that action is more related to other hormones like aldosterone.
- **Option B:** This option is incorrect as ADH does not decrease glomerular filtration rate directly; its primary action is on the collecting ducts.
- **Option C:** This option is incorrect because, although ADH does have a **vasoconstrictive effect** (through V1 receptors) which can increase blood pressure, its primary function related to water balance is not described here.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **diabetes insipidus**, a condition characterized by the inability to produce or respond to ADH, leads to **polyuria** (production of abnormally large volumes of urine) and **polydipsia** (excessive thirst). This condition can be central (lack of ADH production) or nephrogenic (kidney's inability to respond to ADH).

## **Correct Answer:** D. Increases water reabsorption in collecting ducts.