After several days of sustained metabolic acidosis, which adaptive change contributes the largest increase in net acid excretion by the kidney?

• A Increased titratable acid excretion as phosphate availability rises
• B Increased bicarbonate reabsorption in the cortical collecting duct
• C Induction of proximal tubular glutaminase with a marked rise in ammonium excretion ✓
• D Enhanced passive hydrogen ion secretion in the thin descending limb

Explanation

Chronic acidosis upregulates glutamine uptake and glutaminase activity in proximal tubular cells, increasing ammoniagenesis. Ammonium excretion can rise several fold, from roughly 30 to more than 200 mmol per day, making it quantitatively the dominant adaptive response. Titratable acid is limited by the fixed daily supply of filtered phosphate, so it cannot expand comparably, and the thin descending limb plays no secretory role in acid handling.

Reference: Vander's Renal Physiology, 9th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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