## **Core Concept**
Desmopressin is a synthetic analogue of vasopressin (antidiuretic hormone, ADH) used to treat central diabetes insipidus (CDI), a condition characterized by the insufficient production of ADH, leading to an inability to concentrate urine. The action of desmopressin is to mimic ADH, promoting water reabsorption in the collecting ducts of the kidneys.

## **Why the Correct Answer is Right**
In central diabetes insipidus, the kidneys are unable to concentrate urine due to a lack of ADH. Desmopressin acts on the V2 receptors in the collecting ducts of the kidneys, increasing the insertion of aquaporin-2 water channels into the apical membrane, which enhances water reabsorption. A therapeutic response to desmopressin is indicated by an increase in urine osmolality, reflecting the ability to concentrate urine. A more than twofold increase in urine osmolality is generally considered a positive response, indicating effective action of desmopressin.

## **Why Each Wrong Option is Incorrect**
- **Option A:** < 50% increase - This is not a significant response to desmopressin, as a therapeutic effect is typically marked by a substantial increase in urine osmolality. - **Option B:** 10-fold increase - Although a significant increase in urine osmolality is expected, a more than 10-fold increase might not be necessary or typical for all patients, as the goal is to achieve normal or near-normal urine concentration.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that a twofold increase in urine osmolality after administration of desmopressin is often used as a criterion to diagnose central diabetes insipidus and to confirm the effectiveness of desmopressin therapy. This increase indicates that the kidneys are responsive to ADH (or its analogue), helping to differentiate CDI from nephrogenic diabetes insipidus.

## **Correct Answer:** . >2-fold increase.