A patient with type 4 RTA secondary to diabetic nephropathy has a serum K⁺ of 6.1 mEq/L, serum HCO₃⁻ of 18 mEq/L, and urine pH of 5.1. The primary pathophysiological mechanism is reduced aldosterone effect leading to impaired collecting duct acidification. What is the key buffer in the urine that fails to be adequately generated in this condition?

• A Phosphate (titratable acid); impaired because dietary phosphate restriction reduces its filtration
• B Bicarbonate; impaired reabsorption by alpha-intercalated cells in the collecting duct
• C Creatinine; reduced creatinine secretion impairs buffering capacity
• D Ammonium (NH₄⁺); impaired because hyperkalemia inhibits proximal tubule ammoniagenesis ✓

Explanation

In type 4 RTA, the predominant defect is reduced net acid excretion due to impaired ammonium (NH₄⁺) production. Aldosterone deficiency or resistance causes hyperkalemia; elevated extracellular K⁺ competes with H⁺ and NH₄⁺ for tubular transporters and, more importantly, directly inhibits proximal tubule ammoniagenesis from glutamine (via inhibiting glutaminase and α-ketoglutarate dehydrogenase activity). NH₄⁺ is the major urinary buffer for daily acid load (~50–60 mEq/day); its reduction leads to positive H⁺ balance and metabolic acidosis despite the urine pH being appropriately acidic (< 5.5).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.