A common cause of acute interstitial nephritis and a supporting urinalysis finding is?

• A. Drug hypersensitivity; eosinophils in the urine
• B. Bacteria in the urine
• C. Calcium oxalate crystals
• D. WBC casts without eosinophils

Answer: (A)

Drug-induced hypersensitivity is a classic trigger for acute interstitial nephritis, because the immune reaction in the kidney interstitium recruits eosinophils. When eosinophils appear in the urine, eosinophiluria, it supports that the interstitial inflammation is driven by a hypersensitivity reaction rather than by infection or stone-related processes. Bacteria in the urine would point to an infection rather than AIN, calcium oxalate crystals suggest crystal-induced nephropathy or stones, and while white blood cell casts can appear in interstitial nephritis, the presence of eosinophils in the urine provides a more specific link to drug-induced AIN.