## **Core Concept**
Desmopressin acetate (DDAVP) is a synthetic analogue of vasopressin, also known as antidiuretic hormone (ADH). It plays a crucial role in regulating water balance in the body. DDAVP is primarily used to treat central diabetes insipidus and nocturnal enuresis (bedwetting).

## **Why the Correct Answer is Right**
The correct mechanism of action of DDAVP is that it increases water reabsorption in the collecting ducts of the kidneys. This action mimics the effect of natural ADH, which binds to V2 receptors in the collecting ducts, stimulating the insertion of aquaporin-2 water channels into the apical membrane of collecting duct cells. As a result, water permeability increases, allowing more water to be reabsorbed back into the bloodstream, thus concentrating the urine and reducing its volume.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because DDAVP does not work by decreasing the bladder capacity; rather, it acts on the kidneys to decrease urine production.
- **Option B:** This option is incorrect as DDAVP does not primarily act by increasing the bladder muscle tone; its action is on the kidneys, not the bladder.
- **Option D:** This option is incorrect because DDAVP does not decrease fluid intake; instead, it helps the body manage fluids more efficiently by reducing urine output.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that DDAVP is particularly useful in treating nocturnal enuresis because it helps reduce nighttime urine production, thereby decreasing the likelihood of bedwetting. It's also worth noting that DDAVP can be administered intranasally or orally, making it convenient for patients.

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