A 58-year-old heavy smoker presents with weight loss and hyponatremia (serum sodium 118 mEq/L). CT shows a large hilar mass with extensive mediastinal lymphadenopathy. Biopsy shows sheets of small round blue cells with scant cytoplasm, finely granular chromatin, and abundant mitoses. The electrolyte abnormality results from ectopic secretion of which hormone?
- A Parathyroid hormone related peptide
- B Antidiuretic hormone ✓
- C Calcitonin
- D Erythropoietin
Correct answer: B. Antidiuretic hormone
Explanation
Small cell carcinoma is a neuroendocrine tumor that commonly produces paraneoplastic syndromes. Ectopic ADH secretion causes the syndrome of inappropriate antidiuretic hormone, presenting as euvolemic hyponatremia, matching this patient's sodium of 118 mEq/L. PTHrP causing hypercalcemia is typical of squamous cell carcinoma, so option A is the classic distractor killed by the low sodium rather than high calcium.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.