The patient has a non-anion gap hyperchloremic metabolic acidosis and recurrent nephrolithiasis consistent with type 1 RTA.
Type 1 RTA is associated with lupus and other autoimmune disorders such as Sjogren’s syndrome. An inability to secrete H+ at the distal tubule leads to an inability to acidify the urine and the increased excretion of the positive ions calcium and phosphate. Increased excretion of calcium and phosphate into alkaline urine in turns leads to nephrolithiasis. Hypokalemia is also present.
Goldfarb reviews causes of recurrent nephrolithiasis, and recommends that type 1 RTA be suspected in stone-formers with high urinary pH and low serum bicarbonate.
Nicoletta reviews the mechanism of the different RTAs. In type 1 RTA, the urine anion gap is often positive despite a normal serum anion gap due to a marked decrease in urinary NH4+ excretion.
Figure A shows a 3mm calcium stone in the right ureter, as indicated by the black arrow.
Illustration A goes over the characteristics of the various RTAs.
Answers 2 and 3: Recurrent nephrolithiasis is unique feature of type 1 RTA and helps separate type 1 from the other renal tubular acidoses.
Answer 4: Hartnup syndrome is a congenital disorder marked by decreased renal absorption of neutral amino acids.
Answer 5: Fanconi’s syndrome is a congenital or acquired disorder marked by severe proximal tubule dysfunction.
Goldfarb DS, Coe FL. Prevention of recurrent nephrolithiasis. Am Fam Physician. 1999 Nov 15;60(8):2269-76. Review
PMID:10593318 (Link to Abstract)
Nicoletta JA, Schwartz GJ. Distal renal tubular acidosis. Curr Opin Pediatr. 2004 Apr;16(2):194-8. Review
PMID:15021201 (Link to Abstract)