A 32-year-old woman has passed large volumes of pale urine for four months and drinks about eight litres of water each day, having decided this would improve her health. She wakes twice a night to pass urine. Her weight is steady, her blood pressure is 112/70 mmHg and she has no oedema. Urine testing shows no protein or blood. She is admitted for supervised fluid deprivation.
On admission: sodium 133 mmol/L, potassium 3.3 mmol/L, urea 3.4 mmol/L, glucose 5.2 mmol/L, calcium 2.35 mmol/L, plasma osmolality 275 mmol/kg. After eight hours without fluid her urine osmolality is 430 mmol/kg.
Which one of the following best accounts for this woman's urine osmolality after fluid deprivation?