A 58-year-old man with hypertension and chronic kidney disease (GFR 25 mL/min) is prescribed a diuretic for volume overload. Which feature best distinguishes potassium-sparing diuretics from loop diuretics in this clinical context?

Question

Accepted Answer

A. Potassium-sparing diuretics block aldosterone or ENaC in the collecting duct; loop diuretics block NKCC2 in the thick ascending limb. ## Mechanism & Clinical Distinction

**Key Point:** Potassium-sparing and loop diuretics differ fundamentally in their site of action, mechanism, and potassium-handling profile—critical in renal failure.

### Comparative Pharmacology

| Feature | Potassium-Sparing Diuretics | Loop Diuretics |
|---------|---|---|
| **Site of action** | Collecting duct | Thick ascending limb (TAL) |
| **Mechanism** | Aldosterone antagonism (spironolactone) or ENaC blockade (amiloride, triamterene) | Na-K-2Cl cotransporter (NKCC2) inhibition |
| **Effect on K⁺** | **Hyperkalemia** (K⁺ retention) | **Hypokalemia** (K⁺ wasting) |
| **Potency** | Low (collecting duct reabsorbs only 2% of filtered Na⁺) | High (TAL reabsorbs ~25% of filtered Na⁺) |
| **Safety in CKD/GFR < 30** | **CONTRAINDICATED** (risk of life-threatening hyperkalemia) | **Safe and effective** (secreted into tubular lumen) |

### Clinical Context in This Patient

**High-Yield:** In a patient with GFR 25 mL/min, potassium-sparing diuretics are **contraindicated** because the kidneys cannot excrete retained potassium, leading to dangerous hyperkalemia. Loop diuretics remain the drug of choice because they:
1. Are potent enough to overcome reduced GFR
2. Cause hypokalemia (which may be beneficial if baseline K⁺ is high)
3. Work via tubular secretion, bypassing the need for glomerular filtration

**Clinical Pearl:** The collecting duct is the final "fine-tuning" segment for sodium reabsorption. Blocking it (potassium-sparing diuretics) has minimal natriuretic effect but profound potassium-retaining effects—dangerous in renal failure.

**Warning:** A common exam trap is to confuse the *mechanism* (aldosterone blockade) with the *clinical outcome* (hyperkalemia risk). Students may think "aldosterone antagonism is good for heart failure" and forget that it is contraindicated when GFR is severely reduced.

Answer

B. Potassium-sparing diuretics are preferred first-line agents for acute pulmonary edema; loop diuretics are reserved for chronic heart failure

Answer

C. Potassium-sparing diuretics cause hyperkalemia and are contraindicated in renal failure; loop diuretics cause hypokalemia and remain safe

Answer

D. Potassium-sparing diuretics are more potent than loop diuretics in inducing natriuresis

A 58-year-old man with hypertension and chronic kidney disease (GFR 25 mL/min) is prescribed a diuretic for volume overload. Which feature best distinguishes potassium-sparing diuretics from loop diuretics in this clinical context?

Explanation

Practice similar questions