A 70-year-old man with prostate cancer develops confusion, polydipsia, and serum sodium of 122 mEq/L. His urine osmolality is 680 mOsm/kg and serum osmolality is 260 mOsm/kg. Cortisol and thyroid function are normal. This paraneoplastic syndrome is caused by ectopic secretion of:
- A Adrenocorticotropic hormone (ACTH)
- B Antidiuretic hormone (ADH/AVP) ✓
- C Parathyroid hormone-related protein (PTHrP)
- D Erythropoietin
Correct answer: B. Antidiuretic hormone (ADH/AVP)
Explanation
The clinical picture—hyponatremia with inappropriately concentrated urine (high urine osmolality > serum osmolality) with euvolemia—defines SIADH (syndrome of inappropriate ADH secretion). Ectopic ADH production by various tumors (classically SCLC, but also prostate, pancreas) is the most common cause of paraneoplastic SIADH. Ectopic ACTH causes Cushing syndrome (hypertension, hyperglycemia); PTHrP causes hypercalcemia; erythropoietin causes polycythemia.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.