Water Balance and ADH

A 42-year-old woman with a history of recurrent nephrolithiasis presents with polyuria (urine output 4–5 L/day), polydipsia, and serum sodium of 148 mEq/L. Serum osmolality is 305 mOsm/kg and urine osmolality is 150 mOsm/kg. Which investigation is most appropriate to differentiate between central diabetes insipidus (CDI) and nephrogenic diabetes insipidus (NDI)?

Which of the following best describes the primary mechanism by which ADH increases water reabsorption in the collecting duct?

A 58-year-old man with small-cell lung cancer presents with severe hyponatremia (Na+ 118 mEq/L), low serum osmolality (245 mOsm/kg), and urine osmolality of 580 mOsm/kg. Clinical examination shows no signs of volume depletion or edema. Which investigation is most appropriate to confirm the diagnosis of syndrome of inappropriate antidiuretic hormone (SIADH)?

A 58-year-old man with small-cell lung cancer presents with hyponatremia (Na+ 118 mEq/L), low serum osmolality (250 mOsm/kg), and inappropriately elevated urine osmolality (580 mOsm/kg). What is the most common cause of hyponatremia in this clinical scenario?

A 52-year-old man with a history of small-cell lung cancer presents with confusion, lethargy, and a serum sodium of 118 mEq/L. His urine osmolality is 650 mOsm/kg and urine sodium is 85 mEq/L. All of the following are true regarding his condition EXCEPT: