The following Thiazide diuretic is active even if GFR is less than 30-40 mm Hg?
**Core Concept**
The question targets the pharmacokinetics of Thiazide diuretics, specifically their mechanism of action and the factors influencing their efficacy. Thiazide diuretics work by inhibiting the sodium-chloride cotransporter in the distal convoluted tubule, leading to increased sodium and chloride excretion.
**Why the Correct Answer is Right**
The correct answer is a Thiazide diuretic that is effective even in patients with severely impaired renal function, as measured by Glomerular Filtration Rate (GFR). This is because Thiazide diuretics are not significantly renally cleared; instead, they are primarily excreted unchanged in the urine. As a result, their efficacy is not significantly affected by reduced GFR.
**Why Each Wrong Option is Incorrect**
* **Option A:** This Thiazide diuretic is primarily excreted by the kidneys and would be significantly affected by reduced GFR, making it an incorrect choice.
* **Option B:** This Thiazide diuretic is subject to significant tubular secretion, which could be impaired in patients with renal dysfunction, reducing its efficacy.
* **Option C:** This Thiazide diuretic is primarily metabolized by the liver, and its inactive metabolites are excreted by the kidneys. Reduced GFR could affect the excretion of these metabolites, potentially reducing the diuretic's efficacy.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that Thiazide diuretics are effective in patients with reduced renal function because they are not significantly renally cleared. This makes them a useful choice in patients with heart failure, hypertension, or other conditions where diuresis is required, even in the presence of impaired renal function.
**Correct Answer:** B. Metolazone