A patient with heart failure on furosemide develops diuretic resistance. Metolazone is added. The synergistic mechanism of metolazone with loop diuretics is:

• A Metolazone inhibits carbonic anhydrase in the proximal tubule, preventing sodium bicarbonate reabsorption and increasing delivery to Henle's loop
• B Metolazone directly potentiates furosemide binding to NKCC2 in the thick ascending limb by allosteric activation
• C Metolazone inhibits aldosterone secretion, preventing the aldosterone-mediated sodium retention that counteracts furosemide
• D Metolazone inhibits the NCC (Na-Cl cotransporter) in the distal convoluted tubule (DCT), blocking sodium reabsorption at the DCT segment that normally reabsorbs sodium escaping loop diuretic action; this prevents compensatory hypertrophy of DCT and maximizes urinary sodium loss ✓

Explanation

Loop diuretics (furosemide) inhibit NKCC2 in the thick ascending limb (TAL), delivering increased Na+ to the distal nephron. Chronic loop diuretic use induces compensatory hypertrophy of the distal convoluted tubule with upregulation of NCC (sodium chloride cotransporter), which reabsorbs the excess NaCl delivered from the TAL — a major mechanism of diuretic resistance. Metolazone inhibits NCC in the DCT, blocking this compensatory reabsorption. Metolazone uniquely retains activity at low GFR unlike other thiazides (effective even at GFR <30 mL/min), making it the preferred thiazide-like agent for sequential nephron blockade in heart failure. This combination requires careful monitoring for profound electrolyte disturbances.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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