Medullary Sponge Kidney RTA — ESENeph MCQ

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HardRenal Stone DiseaseMedullary Sponge Kidney RTAESENeph

A 45-year-old man with medullary sponge kidney presents with recurrent calcium phosphate stones and persistent urine pH >7.0. Which underlying tubular defect explains this?

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Correct answer: A — Incomplete distal RTA causing impaired urinary acidification

Medullary sponge kidney is associated with incomplete distal (type 1) RTA, which causes persistently alkaline urine (pH >5.5). The alkaline environment promotes calcium phosphate (apatite) stone formation. In complete distal RTA, systemic acidosis develops; in incomplete RTA, the acidification defect is mild and only manifests as inability to lower urine pH, leading to stone formation. Potassium citrate is the treatment of choice to alkalinise urine further (counterintuitive but citrate inhibits crystallisation).

Reference: Gambaro et al 2013 – Medullary Sponge Kidney; EAU 2024 – Urolithiasis