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Cystine stones — RACP Paediatrics MCQ

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ModerateNephrologyCystine stonesRACP Paediatrics

A 5-year-old presents with recurrent renal stones. Stone analysis shows hexagonal crystals. Urine cyanide-nitroprusside test is positive. Both parents are unaffected carriers. What is the most likely stone composition?

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Correct answer: B — Cystine

Hexagonal crystals on urine microscopy and a positive cyanide-nitroprusside test are pathognomonic for cystinuria (autosomal recessive). Cystine stones account for ~5% of paediatric stones. They are radiolucent on plain X-ray but visible on CT. Management includes high fluid intake (to maintain urine output >3 L/m²/day), urine alkalinisation, and D-penicillamine or tiopronin for refractory cases.

Reference: RCH Melbourne – 2023 – Renal Calculi CPG; RACP Paediatric Curriculum – Nephrology