Question
Which of the following statements about Diabetes Insipidus is true:
| A. |
Urine osmolality should be > 300 mosm/L
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| B. |
Plasma osmolality should be < 280 mmol/L
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| C. |
Water deprivation test is required
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| D. |
Plasma osmolality should be > 300 mosm/L prior to H2O Deprivation
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Show Answer
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Correct Answer » C
Explanation
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Correct Answer:
– The *water deprivation test* is the gold standard to diagnose Diabetes Insipidus (DI).
– It distinguishes DI from primary polydipsia by assessing kidney urine-concentrating ability.
– In DI, urine remains dilute despite dehydration, showing low urine osmolality.
Why Option A is Incorrect:
– Urine osmolality in DI is typically *low* (<300 mOsm/kg) due to impaired water reabsorption.
– Urine osmolality >300 mOsm/kg indicates preserved concentration, ruling out DI.
– Typical urine osmolality in DI may be <150 mOsm/kg.
Why Option B is Incorrect:
– Plasma osmolality in DI is usually *elevated* (>295 mOsm/kg) from free water loss and hypernatremia.
– Plasma osmolality <280 mOsm/kg occurs in overhydration states, not DI.
Why Option D is Incorrect:
– Before water deprivation, plasma osmolality is usually normal to mildly elevated.
– Levels >300 mOsm/kg prior to testing are uncommon since thirst keeps osmolality near normal.
– Water deprivation raises plasma osmolality to provoke vasopressin release during the test.
– Therefore, osmolality >300 mOsm/kg occurs during/after deprivation, not before.
