A 35-year-old woman who is being treated for bipolar disorder complains of excessive thirst and frequent urination. She awakens 3-4 times nightly to void. A urinalysis performed after 8 hours of nothing to eat or drink shows the following: Which of the following parts of the nephron is most likely impaired in this patient?

Question

A 35-year-old woman who is being treated for bipolar disorder complains of excessive thirst and frequent urination.  She awakens 3-4 times nightly to void.  A urinalysis performed after 8 hours of nothing to eat or drink shows the following:  Which of the following parts of the nephron is most likely impaired in this patient?

Quizplus · Accepted Answer

This patient presents with polydipsia, polyuria, and an inappropriately low urine specific gravity after 8 hours of water restriction. Although within the normal range, specific gravity should be higher after water deprivation (ie, > 1.010), reflecting concentrated urine. Given her psychiatric history, these findings are highly suggestive of lithium-induced nephrogenic diabetes insipidus. Therapy with lithium reduces the ability of the kidneys to concentrate urine primarily by antagonizing the action of vasopressin (antidiuretic hormone) in the collecting tubules and ducts. Nephrogenic diabetes insipidus caused by lithium usually resolves following discontinuation of the drug. However, impairment can be permanent following years of chronic use. (Choice A) The proximal tubule is the site of action of carbonic anhydrase inhibitors. (Choices B and C) Osmotic diuretics such as mannitol function mainly in the proximal tubule and the descending limb of the loop of Henle. (Choice D) The thick ascending limb of the loop of Henle is the site of action of loop diuretics such as furosemide. (Choice E) The early distal convoluted tubule is the site of action of thiazide diuretics. Educational objective: Lithium-induced diabetes insipidus is the result of lithium's antagonizing effect on the action of vasopressin on principal cells within the collecting duct system. __________ References: Lithium: a versatile tool for understanding renal physiology. (http://www.ncbi.nlm.nih.gov/pubmed/23408166)

A 35-Year-Old Woman Who Is Being Treated for Bipolar Disorder