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Lithium renal adverse effect — MRCPsych Paper A MCQ

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HardPsychopharmacologyLithium renal adverse effectMRCPsych Paper A

A 48-year-old man taking long-term lithium reports marked thirst and passes 6 litres of urine daily. His serum sodium is 150 mmol/L, plasma osmolality is 306 mOsm/kg and urine osmolality is 120 mOsm/kg. His blood glucose is normal. During a supervised diagnostic test, urine osmolality remains below 150 mOsm/kg after desmopressin administration. Which diagnosis best accounts for these findings?

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Correct answer: B — Arginine vasopressin resistance (nephrogenic diabetes insipidus)

The answer is B, arginine vasopressin resistance, traditionally termed nephrogenic diabetes insipidus. Long-term lithium can impair collecting-duct responsiveness to vasopressin, producing large volumes of dilute urine, polydipsia and, when water replacement is insufficient, hypernatraemia and raised plasma osmolality. Failure of urine osmolality to rise after desmopressin identifies renal vasopressin resistance. Central diabetes insipidus would show substantial urine concentration after desmopressin. Primary polydipsia more often produces low or normal plasma sodium and osmolality, with some concentration during water deprivation. Inappropriate antidiuresis causes concentrated urine and hyponatraemia rather than hypotonic polyuria. Diabetes mellitus causes an osmotic diuresis associated with hyperglycaemia and urinary glucose.

Reference: Electronic Medicines Compendium. Lithium Carbonate Essential Pharma 250 mg film-coated tablets, Summary of Product Characteristics, section 4.8 Undesirable effects. Updated 2025. https://www.medicines.org.uk/emc/product/10828/smpc