**Core Concept**
Pituitary diabetes insipidus (DI) is a condition characterized by the deficiency of antidiuretic hormone (ADH) secretion from the posterior pituitary gland. This leads to an inability to concentrate urine, resulting in excessive thirst and polyuria.

**Why the Correct Answer is Right**
The correct answer is desmopressin, a synthetic analogue of ADH. Desmopressin works by stimulating the V2 receptors in the collecting ducts of the kidneys, leading to increased water reabsorption and concentrated urine. This effectively addresses the underlying pathophysiology of pituitary DI.

**Why Each Wrong Option is Incorrect**
**Option A:** Vasopressin is incorrect because, although it is a natural ADH analogue, it has a higher affinity for V1 receptors, which cause vasoconstriction. This can lead to potential side effects like hypertension.
**Option B:** Hydrochlorothiazide is incorrect because it is a diuretic that increases urine production, which would worsen DI symptoms.
**Option C:** Thiazides are incorrect because, although they can be used in nephrogenic DI to increase water reabsorption, they are not the primary treatment for pituitary DI.

**Clinical Pearl / High-Yield Fact**
Desmopressin is a high-yield fact in endocrinology, particularly for treating DI and nocturnal enuresis. It is essential to remember that desmopressin's effectiveness can be reduced in cases of nephrogenic DI.

**Correct Answer: D. Desmopressin**