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Small-cell lung cancer SIADH — SCE Respiratory MCQ

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ModerateLung CancerSmall-cell lung cancer SIADHSCE Respiratory

A 67-year-old man with small-cell lung cancer is admitted with confusion. Sodium is 118 mmol/L, serum osmolality is low and urine osmolality is inappropriately high. He is clinically euvolaemic. What is the most likely underlying cause?

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Correct answer: C — Ectopic antidiuretic hormone secretion

Euvolaemic hyponatraemia with inappropriately concentrated urine in small-cell lung cancer is typical of SIADH from ectopic ADH secretion. PTH-related peptide causes hypercalcaemia, not hyponatraemia. Diabetes insipidus causes dilute urine and hypernatraemia.

Reference: NICE NG122 Lung Cancer; Society for Endocrinology Hyponatraemia Guidance