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Sjogren Type 1 RTA — SCE Rheumatology MCQ

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ModerateConnective Tissue DiseaseSjogren Type 1 RTASCE Rheumatology

A 50-year-old woman with primary Sjögren disease develops muscle weakness. Her serum sodium is 140 mmol/L, potassium 2.8 mmol/L, chloride 116 mmol/L and bicarbonate 15 mmol/L. During systemic acidaemia, her urine pH is 6.2. Urinalysis shows no protein or glucose. Which renal abnormality best explains these findings?

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Correct answer: E — Impaired distal hydrogen-ion secretion causing type 1 renal tubular acidosis

The correct answer is E. The electrolyte results show a normal-anion-gap metabolic acidosis with hypokalaemia. Failure to lower urine pH below 5.5 during systemic acidaemia identifies type 1 distal renal tubular acidosis, caused by impaired hydrogen-ion secretion by type A intercalated cells in the collecting duct. Sjögren disease commonly causes tubulointerstitial renal disease associated with this defect. Type 4 RTA usually causes hyperkalaemia. In type 2 proximal RTA, the distal nephron remains able to acidify urine once plasma bicarbonate has fallen. Fanconi syndrome causes generalised proximal losses, typically including glycosuria despite normal plasma glucose, phosphaturia and aminoaciduria. Nephrotic syndrome would produce heavy proteinuria rather than this tubular acidification defect.

Reference: Wagner CA et al. The pathophysiology of distal renal tubular acidosis. Nature Reviews Nephrology. 2023;19:384–400. https://pubmed.ncbi.nlm.nih.gov/37016093/