Diagnose This (November 19, 2018)

What is your diagnosis? What additional stains would you like to confirm your diagnosis?

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The H&E photomicrograph shows intratubular, lightly pigmented casts with a granular to focally beaded morphology and is suspicious for myoglobin cast nephropathy. For confirmation, myoglobin and hemoglobin immunohistochemical (IHC) stains were ordered (as hemoglobin cast nephropathy has overlapping morphology). By IHC, myoglobin was positive in several intratubular casts while hemoglobin showed no significant staining within the casts. Immunofluorescence showed equal kappa and lambda staining within the casts. Upon speaking with the nephrologist the patient was found down and had a moderately increased serum CK, suspicious for rhabdomyolysis.