A 58-year-old man with small cell lung carcinoma develops hyponatremia with inappropriately elevated urine osmolality. Immunostaining of the tumor would most likely show positivity for which peptide responsible for this paraneoplastic syndrome?

• A Adrenocorticotropic hormone (ACTH)
• B Parathyroid hormone-related protein (PTHrP)
• C Calcitonin
• D Antidiuretic hormone (ADH/AVP) ✓

Explanation

Small cell lung carcinoma (SCLC) commonly causes SIADH through ectopic secretion of antidiuretic hormone (ADH/AVP), producing dilutional hyponatremia with inappropriately concentrated urine. While SCLC can also secrete ACTH causing Cushing syndrome, ectopic ADH production is the most common cause of hyponatremia in SCLC. PTHrP causes hypercalcemia and is more characteristic of squamous cell carcinoma of the lung.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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