A patient with stable chronic metabolic acidosis maintains near-normal acid excretion despite a persistently low serum bicarbonate. Which adaptive mechanism carries the largest share of the additional acid load after several days?

• A Increased titratable acid excretion using filtered phosphate
• B Reduced endogenous acid generation through hepatic urea cycle suppression
• C Enhanced bicarbonate reclamation in the cortical collecting duct via pendrin
• D Upregulated ammonium production from glutamine deamination in the proximal tubule ✓

Explanation

Chronic acidosis induces proximal tubular glutaminase and ammoniagenesis within days, allowing ammonium excretion to rise several-fold and carry most of the added acid load. Titratable acid cannot increase proportionally because it is limited by the fixed amount of filterable buffer such as phosphate. Pendrin mediates bicarbonate secretion, the opposite of what acidosis demands.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.