Concentrating ability of kidney is impaired if:
**Core Concept**
The concentrating ability of the kidney is impaired due to the inability to reabsorb water back into the bloodstream, leading to an excessive loss of water in the urine. This is often seen in conditions where the kidneys are unable to respond to antidiuretic hormone (ADH) or where there is a defect in the collecting ducts.
**Why the Correct Answer is Right**
The concentrating ability of the kidney is impaired if there is a defect in the aquaporin-2 water channels in the collecting ducts. Aquaporin-2 is a protein that helps to reabsorb water back into the bloodstream in response to ADH. In conditions like central diabetes insipidus, where there is a deficiency of ADH, or in nephrogenic diabetes insipidus, where the kidneys are unable to respond to ADH, the aquaporin-2 channels are not activated, leading to an inability to reabsorb water and an excessive loss of water in the urine.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the concentrating ability of the kidney.
* **Option B:** This option is incorrect because it does not directly relate to the concentrating ability of the kidney.
* **Option D:** This option is incorrect because it does not specifically address the concentrating ability of the kidney.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that the concentrating ability of the kidney is impaired in conditions where there is a defect in the aquaporin-2 water channels or in conditions where there is a deficiency or lack of response to ADH.
**Correct Answer: C. Nephrogenic diabetes insipidus.**