Basic and Clinical Sciences (BCSE) Practice Exam

In diabetes insipidus, the primary issue is an inability of the kidneys to concentrate urine due to a lack of adequate levels of antidiuretic hormone (ADH) or the kidneys' resistance to ADH. As a result, there is excessive and dilute urine output, leading to significant losses of free water.

The type of fluid loss in this condition is categorized as pure water loss. This is because the kidneys fail to reabsorb water effectively, leading to a higher volume of urine that is relatively dilute, typically with a low osmolarity. The body loses more water than solutes, hence the term “pure water loss” is descriptive of the fluid imbalance that occurs.

In contrast, isotonic fluid loss would involve a balanced loss of water and solute, hypotonic fluid loss indicates a loss of water with more electrolytes remaining, and hypertonic fluid loss denotes a loss of water that includes a higher concentration of solutes. These descriptions do not align with the nature of fluid loss observed in diabetes insipidus.