**Core Concept**
Diabetes insipidus (DI) is a disorder of antidiuretic hormone (ADH) deficiency or insensitivity, leading to impaired water reabsorption in the collecting ducts of the kidneys. This results in the production of large volumes of dilute urine due to failure of the kidneys to concentrate urine properly.

**Why the Correct Answer is Right**
In diabetes insipidus, the kidneys fail to respond to ADH (vasopressin), which normally acts on the V2 receptors in the collecting ducts to increase aquaporin-2 channel insertion and water reabsorption. Without this, urine becomes highly dilute. The osmolality of urine in DI is typically less than 150 mOsm/L, reflecting a failure to concentrate urine despite high plasma osmolality. This is in contrast to diabetes mellitus, where urine osmolality is high due to hyperglycemia, but the mechanism is different.

**Why Each Wrong Option is Incorrect**
Option B: <300 mmol/L — This range is seen in mild dehydration or normal hydration, not DI. In DI, urine is more dilute than this, so this is too broad and not specific.
Option C: <600 mmol/L — This is typical of moderate dehydration or polyuria in other conditions, but not DI. It does not reflect the extreme dilution seen in DI.
Option D: <900 mmol/L — This is a very broad range and includes normal urine osmolality; it does not capture the hallmark of DI.

**Clinical Pearl / High-Yield Fact**
In diabetes insipidus, urine osmolality is consistently <150 mOsm/kg, and plasma osmolality is elevated. A key diagnostic clue is the presence of polyuria with low urine osmolality and a normal or high serum osmolality. The urine osmolality in DI is often below 100 mOsm/kg, especially in nephrogenic DI.

✓ Correct Answer: A. <150 mmol/L