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Patient behaving strangely, and CNS features were described. Urine osmolality = 1000 and plasma osmolality = 250. What is the most likely diagnosis?
Patient behaving strangely, and CNS features were described. Urine osmolality = 1000 and plasma osmolality = 250. What is the most likely diagnosis?
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Compare plasma and urine osmolality: low plasma osmolality with inappropriately concentrated urine suggests hyponatremia with SIADH.
Updated On: Jun 23, 2026
- Hyponatremia
- Hypernatremia
- Hypokalemia
- Hyperkalemia
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The Correct Option is A
Solution and Explanation
Osmolality-based diagnostic approach:
Given: CNS symptoms (strange behavior), urine osm = 1000 mOsm/kg, plasma osm = 250 mOsm/kg.
Step 1 -- Plasma osmolality interpretation:
Normal plasma osmolality = 285-295 mOsm/kg. Here it is 250 -- significantly low, indicating hypo-osmolar plasma. Plasma osm approximately = 2 x [Na] + glucose/18 + BUN/2.8. Low plasma osm strongly suggests low serum Na (hyponatremia).
Step 2 -- Urine osmolality interpretation:
Urine osm = 1000 mOsm/kg (markedly high = concentrated urine). Expected response to hypo-osmolar plasma: kidneys should suppress ADH and produce dilute urine (osm <100). Instead: high urine osm means ADH is inappropriately elevated = SIADH pattern.
Step 3 -- CNS manifestations of hyponatremia:
Serum Na <125 mEq/L leads to cerebral edema: cells swell due to osmotic water shift, causing altered behavior, confusion, and potentially seizures or coma.
Conclusion: The pattern (low plasma osm + inappropriately high urine osm + CNS features) = Hyponatremia (SIADH).
\[\boxed{\text{Hyponatremia}}\]
Given: CNS symptoms (strange behavior), urine osm = 1000 mOsm/kg, plasma osm = 250 mOsm/kg.
Step 1 -- Plasma osmolality interpretation:
Normal plasma osmolality = 285-295 mOsm/kg. Here it is 250 -- significantly low, indicating hypo-osmolar plasma. Plasma osm approximately = 2 x [Na] + glucose/18 + BUN/2.8. Low plasma osm strongly suggests low serum Na (hyponatremia).
Step 2 -- Urine osmolality interpretation:
Urine osm = 1000 mOsm/kg (markedly high = concentrated urine). Expected response to hypo-osmolar plasma: kidneys should suppress ADH and produce dilute urine (osm <100). Instead: high urine osm means ADH is inappropriately elevated = SIADH pattern.
Step 3 -- CNS manifestations of hyponatremia:
Serum Na <125 mEq/L leads to cerebral edema: cells swell due to osmotic water shift, causing altered behavior, confusion, and potentially seizures or coma.
Conclusion: The pattern (low plasma osm + inappropriately high urine osm + CNS features) = Hyponatremia (SIADH).
\[\boxed{\text{Hyponatremia}}\]
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