Rosh Emergency Medicine Practice Test

What laboratory abnormality best explains the symptoms in a patient experiencing increased urination and thirst after a traumatic brain injury?

1. Plasma osmolality 200 mOsm/kg

2. Serum sodium 152 mEq/L

3. Urine sodium 22 mEq/L

4. Urine specific gravity 1.022

The correct answer is: Serum sodium 152 mEq/L

The symptoms of increased urination and thirst in a patient following a traumatic brain injury are indicative of a condition known as diabetes insipidus. This condition can occur when there is damage to the hypothalamus or pituitary gland, areas responsible for the regulation of antidiuretic hormone (ADH, also known as vasopressin). In the case presented, an elevated serum sodium level of 152 mEq/L provides a significant clue. This level indicates hypernatremia, which is a result of the body losing a large volume of water, either due to inadequate ADH secretion or a lack of kidney responsiveness to ADH. When the serum sodium concentration rises, it signals that there is a relative deficit of water in the body, leading to increased thirst (polydipsia) and urination (polyuria). The high serum sodium correlates with the symptoms described and aligns well with the expected physiological response to diabetes insipidus. In contrast, a plasma osmolality of 200 mOsm/kg would suggest a state of hyponatremia or excess water, not hypernatremia, making it inconsistent with the observed symptoms. A urine sodium of 22 mEq/L would not typically be high if