Drug affecting positive free water clearance without affecting negative free water clearance –
## Core Concept
The question tests the understanding of how different drugs affect free water clearance in the kidneys. Free water clearance refers to the volume of water that is cleared from the plasma per unit time and is an indicator of the kidney's ability to concentrate or dilute urine. Positive free water clearance indicates the excretion of dilute urine, while negative free water clearance indicates the excretion of concentrated urine.
## Why the Correct Answer is Right
The correct answer, **Lithium**, affects positive free water clearance without impacting negative free water clearance. Lithium acts by inhibiting the action of antidiuretic hormone (ADH) at the collecting ducts in the kidneys. ADH, also known as vasopressin, promotes water reabsorption in the collecting ducts, leading to more concentrated urine. By inhibiting ADH's action, lithium reduces water reabsorption, leading to an increase in urine output and a decrease in urine osmolality. This effect is more pronounced when the body is trying to excrete dilute urine (positive free water clearance), as lithium directly interferes with the kidney's ability to respond to ADH.
## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because the specific drug and its effect on free water clearance are not provided, making it impossible to assess its accuracy.
- **Option B:** Similarly, without specific details on the drug's mechanism or effect on free water clearance, it's challenging to evaluate its correctness.
- **Option D:** This option is also incorrect as it lacks specific information about the drug's impact on free water clearance.
## Clinical Pearl / High-Yield Fact
A key point to remember is that lithium can cause nephrogenic diabetes insipidus, a condition characterized by the kidney's inability to respond to ADH, leading to an inability to concentrate urine and resulting in polyuria and polydipsia. This condition directly relates to lithium's effect on positive free water clearance.
## Correct Answer Line
**Correct Answer: C. Lithium**