Free water clearance of -1.3 ml/min signifies
**Core Concept**
Free water clearance (C_H2O) is a measure of the volume of plasma water that is cleared of solute-free water per unit time, typically expressed in mL/min. It is an important parameter in assessing the body's ability to regulate fluid balance and is often used to evaluate the function of the kidneys.
**Why the Correct Answer is Right**
A free water clearance of -1.3 ml/min indicates that the kidneys are reabsorbing water, which is a sign of concentrated urine production and a state of dehydration. This occurs when the antidiuretic hormone (ADH) is present, causing the collecting ducts to reabsorb water back into the bloodstream, thereby concentrating the urine. The negative value indicates that the kidneys are removing water from the urine, rather than adding to it.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a free water clearance of 0 would indicate no net movement of water across the renal tubules, which is not the case here. A value of -1.3 ml/min indicates active water reabsorption.
**Option B:** This option is incorrect because a free water clearance of 1.3 ml/min would indicate free water excretion, which is the opposite of what is happening in this scenario.
**Option C:** This option is incorrect because a free water clearance of -1.3 ml/min does not indicate hypovolemia; rather, it indicates that the kidneys are actively reabsorbing water to concentrate the urine.
**Clinical Pearl / High-Yield Fact**
In clinical practice, a free water clearance of -1.3 ml/min can be seen in patients with dehydration, especially those with diabetes insipidus or nephrogenic diabetes insipidus.
**Correct Answer:** C.