**Core Concept**
Lithium-induced diabetes insipidus (DI) is a complication of lithium treatment, characterized by an impaired renal response to ADH (antidiuretic hormone), leading to excessive urine production and polyuria. This condition is distinct from diabetes mellitus, which is a metabolic disorder resulting from insulin deficiency or resistance.

**Why the Correct Answer is Right**
The drug of choice in lithium-induced diabetes insipidus is desmopressin, a synthetic analogue of vasopressin (ADH). Desmopressin works by binding to V2 receptors in the renal collecting ducts, stimulating the insertion of aquaporin-2 water channels and increasing water reabsorption. This leads to a decrease in urine output and an improvement in symptoms. The use of desmopressin is effective in managing lithium-induced DI, as it bypasses the impaired renal response to ADH and directly stimulates water reabsorption.

**Why Each Wrong Option is Incorrect**
**Option A:** Insulin would be ineffective in treating lithium-induced DI, as the underlying issue is not insulin deficiency or resistance, but rather an impaired renal response to ADH.
**Option B:** Metformin, a biguanide, is used to treat type 2 diabetes mellitus by decreasing glucose production in the liver and increasing insulin sensitivity. It would not be effective in treating lithium-induced DI.
**Option C:** Glipizide, a sulfonylurea, stimulates insulin release from the pancreas and would be ineffective in treating lithium-induced DI.
**Option D:** Thiazide diuretics, such as hydrochlorothiazide, can be used to treat lithium-induced DI, but they work by increasing sodium reabsorption in the distal convoluted tubule, leading to a decrease in urine production. However, they are not the first-line treatment and may have limited efficacy compared to desmopressin.

**Clinical Pearl / High-Yield Fact**
It is essential to differentiate between lithium-induced diabetes insipidus and diabetes mellitus, as the treatment approaches are distinct. Lithium-induced DI can be challenging to diagnose, as it may present with symptoms similar to diabetes mellitus, such as polyuria and polydipsia.

**Correct Answer: D. Hydrochlorothiazide.**