This biopsy is a 54-year-old female without significant past medical history, who presented with acute kidney injury. The serum creatinine at presentation was 7.2 mg/dl. Light microscopy shows diffuse acute tubular injury characterized by marked attenuation of the tubular epithelium with loss of the proximal tubular brush borders (not shown). Additionally, the tubular lumens frequently show globular, retractile, brightly eosinophilic and fuchsinophilic casts, without an associated cellular reaction (Fig 1, PAS; Fig 2, Trichrome). The differential diagnosis of acute tubular injury with pigmented casts includes hemoglobin cast nephropathy, myoglobin cast nephropathy, bile cast nephropathy, light chain cast nephropathy and acute tubular injury with abundant intratubular cellular debris. In this case, the globular configuration of the casts, along with its tinctorial characteristics, is strongly suggestive of myoglobin casts; however, these findings are neither sensitive nor specific. A myoglobin immunoperoxidase stain was positive within the casts (Fig 3), confirming the diagnosis of myoglobin cast nephropathy.
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