What is your diagnosis?
This photomicrograph of an H&E section shows an area of low corticomedullary junction with numerous calcium phosphate crystals within tubules. Calcium phosphate crystals appear deep purple on H&E, black on Von Kossa special stain and show minimal birefringence on polarization. The density of calcium phosphate deposition in the photomicrograph is consistent with nephrocalcinosis.
Nephrocalcinosis has a variable clinical presentation depending on the etiology but often presents as chronic kidney disease with minimal proteinuria and a benign urine sediment and is typically seen in the setting of hypercalcemia or hyperphosphatemia which produces a large list of potential etiologies (phosphate nephropathy from oral sodium phosphate ingestion [old bowel purgative solutions], Dent disease, medullary sponge kidney, nephrogenic systemic fibrosis, inherited tubulopathies, sarcoidosis, hypervitaminosis A or D, malignancy, milk-alkali syndrome, chronic hypercalcemia of unknown etiology, etc.). Treatment is typically directed at the underlying cause and prognosis varies. Interestingly, at least one study has shown that specific morphologic patterns of calcium phosphate crystals in the kidney biopsy correlate with either a hyperphosphaturic or hypercalciuric etiology (see reference).
Reference: Wiech T, Hopfer H, Gaspert A, et al. Histopathological patterns of nephrocalcinosis: a phosphate type can be distinguished from a calcium type. Nephrol Dial Transplant (2012) 27: 1122-1131.
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