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Cystinuria Sodium Restriction Cystine Excretion — ESENeph MCQ

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ModerateRenal Stone DiseaseCystinuria Sodium Restriction Cystine ExcretionESENeph

A 30-year-old man has recurrent renal stones. Metabolic evaluation reveals elevated urine cystine, ornithine, lysine, and arginine. Urine cystine excretion is 2200 umol/day (markedly elevated). He is already on high fluid intake (>3 L/day) and potassium citrate (urine pH 7.0). Tiopronin has been started. What dietary modification is also recommended?

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Correct answer: C — Low-sodium diet (sodium restriction reduces cystine excretion)

In cystinuria, sodium restriction reduces urinary cystine excretion. The mechanism involves coupled sodium and cystine reabsorption in the proximal tubule — reducing sodium delivery to the proximal tubule (by restricting dietary sodium) enhances proximal sodium reabsorption and secondarily increases cystine reabsorption, reducing cystine in the final urine. Sodium intake should be limited to <100 mmol/day (<2.3 g/day). Additionally, excessive animal protein intake increases cystine excretion and should be moderated. The combination of high fluid intake (target urine output >3 L/day), urine alkalinisation (potassium citrate targeting pH 7.0-7.5), sodium restriction, moderate protein intake, and thiol agents (tiopronin) forms the complete management package.

Reference: Goldfarb et al 2006 – Cystinuria Management; EAU 2023 – Urolithiasis Guidelines