Renal tubular acidosis type I (distal RTA) is characterized by inability to lower urine pH below 5.5 despite systemic acidosis. The primary defect is in:
- A Carbonic anhydrase II deficiency in collecting duct intercalated cells
- B Impaired NHE3 (Na+/H+ exchanger) in the proximal tubule
- C Aldosterone resistance in principal cells reducing H+ secretion
- D Defective H+-ATPase or anion exchanger (AE1/Cl-/HCO3- exchanger) in alpha-intercalated cells of the collecting duct, preventing adequate luminal acidification ✓
Explanation
Type I (distal) RTA results from defective H+ secretion by alpha-intercalated cells in the collecting duct. These cells secrete H+ via luminal H+-ATPase (V-type) and H+/K+-ATPase, supported by basolateral Cl-/HCO3- exchanger (AE1, encoded by SLC4A1). Genetic defects in H+-ATPase (ATP6V1B1, ATP6V0A4) or AE1 are the most common causes. The consequence is inability to lower urine pH below 5.3–5.5, leading to systemic hyperchloremic, hypokalemic metabolic acidosis and nephrocalcinosis. Carbonic anhydrase II deficiency causes a mixed proximal and distal RTA (Type III). NHE3 impairment causes proximal (Type II) RTA. Aldosterone resistance primarily causes Type IV (hyperkalemic) RTA.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.