**Core Concept**
Central diabetes insipidus is a condition characterized by the inadequate production of antidiuretic hormone (ADH), also known as vasopressin, leading to an inability to regulate water balance in the body. This results in excessive thirst and polyuria (excessive urine production). The underlying pathophysiology involves impaired ADH secretion from the posterior pituitary gland.

**Why the Correct Answer is Right**
In central diabetes insipidus, the body lacks sufficient ADH to stimulate water reabsorption in the kidneys. As a result, the urine osmolality is decreased, reflecting the kidneys' inability to concentrate urine. This is because ADH acts on the collecting ducts in the kidneys to increase water reabsorption, thereby increasing urine osmolality. In this patient, the urine osmolality is expected to be low, typically < 150 mOsm/kg. **Why Each Wrong Option is Incorrect** **Option A:** This option is incorrect because it represents a normal urine osmolality range, which would be expected in a patient without diabetes insipidus. Patients with central diabetes insipidus typically have low urine osmolality. **Option B:** This option is incorrect because it represents a high urine osmolality range, which would be expected in a patient with nephrogenic diabetes insipidus, a condition where the kidneys are unable to respond to ADH. **Clinical Pearl / High-Yield Fact** In central diabetes insipidus, the plasma osmolality is typically elevated, reflecting the body's inability to regulate water balance. This can be distinguished from the plasma osmolality in nephrogenic diabetes insipidus, which is typically normal. **Correct Answer:** C. Low urine osmolality.