A 58-year-old man with small cell lung carcinoma develops hyponatremia, concentrated urine with urine osmolality 620 mOsm/kg, and serum osmolality 265 mOsm/kg. Brain MRI is normal. Which ectopic peptide best explains this paraneoplastic syndrome?
- A ADH (arginine vasopressin) ✓
- B ACTH
- C PTHrP
- D Calcitonin
Correct answer: A. ADH (arginine vasopressin)
Explanation
Small cell lung carcinoma is the classic cause of ectopic ADH (SIADH) secretion, producing dilutional hyponatremia with inappropriately concentrated urine. PTHrP causes hypercalcemia, ACTH causes Cushing syndrome, and calcitonin lowers calcium — none fit the clinical picture of euvolemic hyponatremia with concentrated urine.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.