**Core Concept**
Diabetes insipidus (DI) is a disorder characterized by excessive thirst and polyuria due to an inability to regulate water levels in the body. It can be caused by either a deficiency of antidiuretic hormone (ADH) or a problem with the kidneys' ability to respond to ADH.

**Why the Correct Answer is Right**
The treatment of choice for DI involves drugs that either replace or mimic ADH. Desmopressin, a synthetic analogue of ADH, is commonly used to treat central DI. Chlorpropamide, a sulfonylurea, is used to treat nephrogenic DI by increasing the responsiveness of the kidneys to ADH. Hydrochlorothiazide, a thiazide diuretic, is also used to treat nephrogenic DI by reducing the amount of sodium delivered to the distal tubules, which in turn increases the responsiveness of the kidneys to ADH.

**Why Each Wrong Option is Incorrect**
**Option A:** Vasopressin is actually the same as desmopressin, a synthetic analogue of ADH. It is used to treat DI, making it an incorrect choice.
**Option B:** Hydrochlorothiazide is actually used to treat nephrogenic DI, making it an incorrect choice.
**Option C:** This option is missing, so I will provide one: Metformin is an oral antidiabetic drug used to treat type 2 diabetes mellitus, not DI.

**Clinical Pearl / High-Yield Fact**
In patients with DI, the administration of a thiazide diuretic can actually decrease urine output and improve symptoms, even though it is a diuretic.

**Correct Answer:** A. Vasopressin