A 58-year-old man with small cell lung carcinoma develops hyponatremia and inappropriately concentrated urine. Immunohistochemistry of the tumor shows strong positivity for chromogranin A and synaptophysin. The paraneoplastic syndrome in this patient is mediated by ectopic secretion of which peptide?

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

Explanation

Small cell lung carcinoma (SCLC) is the classic tumor causing SIADH via ectopic secretion of ADH/AVP, leading to hyponatremia with inappropriately high urine osmolality. PTHrP causes hypercalcemia of malignancy. Ectopic ACTH causes Cushing's syndrome. Chromogranin A and synaptophysin confirm neuroendocrine differentiation of SCLC.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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