## **Core Concept**
Lithium-induced polyuria is a common side effect of lithium therapy, primarily due to its effect on the kidneys' ability to concentrate urine. This condition arises from lithium's interference with the action of antidiuretic hormone (ADH), also known as vasopressin, leading to nephrogenic diabetes insipidus. The management of this condition involves either reducing polyuria or addressing the underlying cause.

## **Why the Correct Answer is Right**
The correct answer, **Amiloride**, is a potassium-sparing diuretic that acts by blocking epithelial sodium channels in the collecting ducts of the kidneys. In the context of lithium-induced polyuria, amiloride is effective because it can help decrease urine volume without causing significant changes in sodium or potassium balance. Unlike other treatments that might be used for central diabetes insipidus (like desmopressin), amiloride is specifically useful for nephrogenic diabetes insipidus caused by lithium.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **Desmopressin** is a synthetic analogue of vasopressin and is used to treat central diabetes insipidus, it is not effective in lithium-induced nephrogenic diabetes insipidus because the kidneys are unresponsive to ADH.
- **Option B:** **Hydrochlorothiazide** is a thiazide diuretic that, paradoxically, can reduce polyuria in nephrogenic diabetes insipidus by inducing mild volume contraction, which enhances proximal tubular reabsorption of sodium, water, and glucose. However, it is not the first choice due to potential side effects and is less effective than amiloride in some cases.
- **Option D:** **Indomethacin** is a nonsteroidal anti-inflammatory drug (NSAID) that can reduce polyuria by enhancing proximal tubular reabsorption of water. However, its use is limited by potential side effects, especially in patients with renal impairment.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in managing lithium-induced polyuria, **Amiloride** is often considered a first-line treatment due to its efficacy and relatively favorable side effect profile compared to other options. It's also crucial to monitor lithium levels and renal function in patients on long-term lithium therapy.

## **Correct Answer:** . **Amiloride**