**Question:** In a 60-year-old female with polydipsia, water deprivation test was performed and the values obtained are as following - Baseline: Na - 139 mEq, K-3.6mEq. Osm.plasma-274 mosm/l, Osm. urine-150 after vasopressin: Na-142mEq, K-3.9mEq, Plasma osm-271 mosm/l, Urine osm - 489 mosm/l. These findings suggest a diagnosis of -

A. Hyponatremia
B. Hypokalemia
C. Hypernatremia
D. Hypovolemia

**Correct Answer:**

**Core Concept:** The water deprivation test is a diagnostic tool used to evaluate the function of the antidiuretic hormone (ADH), also known as vasopressin. ADH is released by the hypothalamus and acts on the collecting ducts of the kidney, increasing water reabsorption and decreasing urine output. Water deprivation causes a decrease in urine output, which is expected to increase ADH production.

**Why the Correct Answer is Right:** The water deprivation test results show a reduction in urine output (from baseline osmolar urine concentration of 150 mosm/L) following administration of vasopressin (increased urine osmolarity to 489 mosm/L). This indicates a defect in the action of vasopressin on the collecting ducts of the kidney, suggesting a dysfunction in the ADH system.

**Why Other Options are Incorrect:**

A. Hyponatremia: This option is incorrect because the sodium concentration in the blood remains normal (139 mEq) during the water deprivation test, indicating that the issue is not related to sodium levels.

B. Hypokalemia: The potassium concentration remains within normal limits (3.9 mEq) during the test, ruling out hypokalemia as the cause.

C. Hypernatremia: This option is incorrect because the sodium concentration in the blood remains normal (139 mEq) during the test, indicating that the issue is not related to sodium levels.

D. Hypovolemia: Although the water deprivation test is performed to induce dehydration, the decrease in urine output does not necessarily indicate hypovolemia. The core issue is likely a dysfunction in the ADH system, not a volume status.

**Clinical Pearl:**

The water deprivation test is a valuable tool for assessing ADH function in clinical practice, particularly when investigating polyuria (excessive urine output). By comparing the results of the test with or without vasopressin administration, one can differentiate between primary and secondary polyuria, which is essential for accurate diagnosis and appropriate treatment planning.