Question

Patient behaving strangely, and CNS features were described. Urine osmolality = 1000 and plasma osmolality = 250. What is the most likely diagnosis?

• A. Hyponatremia
• B. Hypernatremia
• C. Hypokalemia
• D. Hyperkalemia

Hint:

Compare plasma and urine osmolality: low plasma osmolality with inappropriately concentrated urine suggests hyponatremia with SIADH.

Answer 1

The Correct Option is A

Step 1: Interpret plasma osmolality.
Plasma osmolality = 250 mOsm/kg (normal 285-295 mOsm/kg). This is LOW -- indicating a hypo-osmolar state. Low plasma osmolality with CNS symptoms (confusion, strange behavior) points to hyponatremia.

Step 2: Interpret urine osmolality.
Urine osmolality = 1000 mOsm/kg. High urine osmolality means the kidneys are concentrating urine (ADH is active). In hyponatremia, if the kidneys were responding appropriately, urine should be dilute (<100 mOsm/kg). Here the kidneys are inappropriately concentrating -- this is the pattern of SIADH (Syndrome of Inappropriate ADH secretion).

Step 3: Correlation with symptoms.
Hyponatremia causes neurological symptoms (strange behavior, CNS features, confusion, seizures) because low serum sodium leads to cerebral edema due to osmotic water entry into neurons.

Step 4: Conclusion.
Low plasma osmolality + high urine osmolality + CNS features = Hyponatremia (likely SIADH). The body holds too much water, diluting sodium and causing symptoms including confusion, lethargy, and seizures.