Answer: A, 2,8-Dihydroxyadeninuria
2,8-dihydroxyadeninuria (2,8-DHA) is caused by an autosomal recessive deficiency in the adenine phosphoribosyltransferase (APRT). The gene is located on chromosome 16q24 of which greater than forty different mutations have been identified which include missense, nonsense, insertion, and deletion mutations. APRT is a purine salvage enzyme which in disease states leads to the absence of recycling adenine. Instead, adenine is eventually catabolized to 2,8-DHA which, in the kidney, leads to crystalline and stone formation due to its poor solubility in the urine. By light microscopy, these crystals are red to brown and birefringent on H&E and PAS stains while appear black on silver stain. Of note, the differential also includes triamterene crystalline nephropathy.
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