A 69-year-old African American woman had a kidney biopsy due to CKD III-IV. Her medical history is significant for morbid obesity, diabetes mellitus (>5 years), hypertension (>5 years), coronary artery disease status post stent, hypothyroidism, gout and chronic kidney disease. Her renal ultrasound shows an unremarkable simple left renal cyst and increased echogenicity in both kidneys. Investigations show serum creatinine which has increased to 4.9 mg/dl. Proteinuria is up to 3.7 gm/day. Serological studies are negative.
The images are characteristic findings of which of the following:
2. Acute Tubular Injury
3. Uric Acid Nephropathy
4. Interstitial Nephritis
5. Indinavir toxicity
The correct answer is Uric Acid Nephropathy. The images show micro tophi within the interstitium. These are seen as amorphous areas or poorly formed granulomas containing needle-shaped clefts and surrounded by clusters of macrophages, giant cells, and lymphocytes. Urate crystals are dissolved during formalin fixation, leading to the empty needle-shaped spaces. However, if the sections are not formalin fixed, such as the frozen sections used for immunofluorescence, these crystals can occasionally be seen as birefringent, needle-shaped crystals on polarization (Fig 5). The deposits are usually monosodium urate monohydrate or rarely ammonium urate. Patients can present with acute uric acid nephropathy (usually in the context of severe hyperuricemia of >15 mg/dL), chronic uric acid nephropathy (such as in patients with gout) or uric acid calculi. Sarcoid granulomas are well formed, non-caseating and often do not contain needle-shaped empty spaces. Interstitial nephritis usually has more inflammation, tubulitis and is often rich in eosinophils. Needle shaped crystals can be seen with antiviral medications too such as indinavir, but these are typically within the tubules and there is no history of antiviral therapy in this patient.
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