Renal tubular acid–base disorders that cause normal anion gap metabolic acidosis due to impaired hydrogen ion secretion include which types?

• A. Proximal RTA Type II
• B. Distal RTA Type I and Type IV RTA
• C. Nephrotic syndrome
• D. Acute tubular necrosis

Answer: (B)

Normal anion gap metabolic acidosis from impaired hydrogen ion secretion happens when the distal nephron cannot excrete H+ effectively, so acid accumulates while the anion gap stays normal. This is the hallmark of distal RTA, specifically Type I and Type IV. In Type I distal RTA, the collecting duct cells fail to secrete H+ properly, so urine remains inappropriately alkaline (high pH) despite systemic acidosis, and patients may develop nephrocalcinosis or stones and hypokalemia. In Type IV RTA, aldosterone deficiency or resistance reduces ammonium (NH4+) production and excretion—the main route of distal hydrogen ion elimination—leading to hyperchloremic acidosis often with hyperkalemia. Proximal RTA causes bicarbonate loss from the proximal tubule, not impaired H+ secretion, so its acidosis is not due to defective distal hydrogen ion secretion. Nephrotic syndrome and acute tubular necrosis cause acidosis mainly from reduced overall kidney function, not from a primary defect in H+ secretion.