In acute interstitial nephritis, which finding is considered pathognomonic?

In acute interstitial nephritis, which finding is considered pathognomonic?

• A. Eosinophilia
• B. WBC casts
• C. Fever
• D. Maculopapular rash

Answer: (B)

The key idea is that acute interstitial nephritis is most strongly suggested by eosinophils in the urine. Eosinophiluria, detected on a Hansel stain, reflects an allergic interstitial inflammatory process in the kidney and is considered highly characteristic of drug-induced AIN. Peripheral eosinophilia, fever, and rash can occur with AIN but are not specific to it. White blood cell casts may be seen with interstitial inflammation as well, but they are not unique to AIN and can occur in other kidney conditions. So, while eosinophiluria is the definitive clue, the listed options don’t include that exact finding; among them, white blood cell casts are the most supportive for AIN, but the true hallmark remains eosinophils in the urine.

The key idea is that acute interstitial nephritis is most strongly suggested by eosinophils in the urine. Eosinophiluria, detected on a Hansel stain, reflects an allergic interstitial inflammatory process in the kidney and is considered highly characteristic of drug-induced AIN. Peripheral eosinophilia, fever, and rash can occur with AIN but are not specific to it. White blood cell casts may be seen with interstitial inflammation as well, but they are not unique to AIN and can occur in other kidney conditions. So, while eosinophiluria is the definitive clue, the listed options don’t include that exact finding; among them, white blood cell casts are the most supportive for AIN, but the true hallmark remains eosinophils in the urine.