Interstitial nephropathy (also known as tubulointerstitial nephritis) is most commonly an allergic-type reaction to medications, typified by eosinophils in the urine. The nonspecific maculopapular reaction is also the common type of drug reaction rash and is seen in a minority of cases, as is low-grade fever (not critical to the question). Several medications can cause this: penicillins, cephalosporins, and NSAIDs are the most common.