An 83-year-old woman presents to your office with a 2-day history of confusion.

Comprehensive and Detailed Explanation:

Hyponatremia in a patient with lung cancer and euvolemia strongly suggests syndrome of inappropriate antidiuretic hormone secretion (SIADH), especially from small cell carcinoma. Serum osmolality is the best initial test to confirm hypotonic hyponatremia and distinguish true hyponatremia from pseudohyponatremia or other causes.

Toronto Notes 2023 – Endocrinology, “Hyponatremia”:

“Serum osmolality helps classify hyponatremia as hypotonic, isotonic, or hypertonic. SIADH typically causes hypotonic hyponatremia in euvolemic patients.”

MCCQE1 Objectives (Endocrinology > 37-1: Electrolyte Disorders):

“Candidates must evaluate the type and cause of hyponatremia using clinical status and laboratory tests including serum osmolality.”

Urine sodium (B) is useful after confirming hypotonicity. PTHrP (E) is associated with hypercalcemia of malignancy, not hyponatremia. Urinalysis (A) and CrCl (D) are less directly informative.