Following is true about furosemide –
**Core Concept**
Furosemide is a loop diuretic that acts on the Loop of Henle in the kidneys to increase urine production. Its mechanism of action involves inhibiting the sodium-potassium-chloride cotransporter (NKCC2) in the thick ascending limb of the Loop of Henle.
**Why the Correct Answer is Right**
Furosemide blocks the NKCC2 cotransporter, preventing the reabsorption of sodium, potassium, and chloride ions in the Loop of Henle. This leads to an increase in the amount of water, sodium, and chloride in the urine, resulting in a potent diuretic effect. The inhibition of NKCC2 also leads to a decrease in the level of intracellular chloride, which in turn inhibits the calcium-sensing receptor (CaSR) and reduces the reabsorption of calcium in the distal convoluted tubule.
**Why Each Wrong Option is Incorrect**
**Option A:** Furosemide is primarily excreted by the liver. **Incorrect because** furosemide is mainly excreted by the kidneys, with a small amount being excreted in the bile.
**Option B:** Furosemide has a narrow therapeutic index. **Incorrect because** furosemide has a relatively wide therapeutic index, allowing for some variation in dosage without causing significant toxicity.
**Option C:** Furosemide is a potassium-sparing diuretic. **Incorrect because** furosemide is a loop diuretic that actually increases potassium excretion in the urine.
**Clinical Pearl / High-Yield Fact**
It's essential to note that furosemide can cause a significant increase in serum potassium levels in patients with renal failure or those taking potassium-sparing medications. This highlights the importance of monitoring potassium levels in these patients and adjusting their treatment accordingly.
**Correct Answer:** D.