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MBBS QUESTION #10263
Question 1
A patient with a small cell lung carcinoma develops hyponatremia due to syndrome of inappropriate antidiuretic hormone secretion (SIADH). The hyponatremia is best explained by:
  • Unregulated ADH action causing excessive free water reabsorption in the collecting duct, producing dilutional hyponatremia with concentrated urine despite low plasma osmolality✔️
  • Excessive urinary sodium loss due to aldosterone deficiency
  • Reduced ADH secretion causing polyuria and dilute urine
  • Primary loss of extracellular fluid volume from vomiting
Correct Answer Explanation
In SIADH, ADH is secreted independent of normal osmotic or volume control (often from an ectopic tumor source), causing continued water reabsorption despite low plasma osmolality — producing euvolemic dilutional hyponatremia with inappropriately concentrated urine.