Question
In a patient with diabetes insipidus, which of the following findings is expected?
| A. |
A. High urine specific gravity
|
| B. |
B. Hyponatremia
|
| C. |
C. Elevated serum osmolality
|
| D. |
D. Low serum sodium
|
Show Answer
|
Correct Answer » C
Explanation
|
|
Diabetes insipidus (DI) is characterized by deficient secretion of antidiuretic hormone (ADH) (central DI) or renal resistance to ADH (nephrogenic DI). As a result, the kidneys are unable to concentrate urine, leading to excessive loss of free water.
This causes:
- Polyuria
- Polydipsia
- Dilute urine with low urine osmolality and low urine specific gravity
- Increased serum osmolality
- Hypernatremia if water intake is inadequate
Therefore, elevated serum osmolality is the expected laboratory finding.
Why the Other Options Are Incorrect
A. High urine specific gravity
- Incorrect.
- Patients with diabetes insipidus produce very dilute urine, resulting in low urine specific gravity (typically <1.005).
B. Hyponatremia
- Incorrect.
- Diabetes insipidus causes excessive free water loss, leading to hypernatremia, not hyponatremia, unless water intake is excessive.
D. Low serum sodium concentration
- Incorrect.
- Serum sodium is usually normal or elevated due to water loss.
- Low serum sodium is more characteristic of Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH).
