A 70-year-old man with type 2 diabetes and hypertension presents with a 2-week history of fatigue, malaise, and a new rash. He recently started taking a new antibiotic for a presumed urinary tract infection. His creatinine has risen from 1.2 mg/dL to 3.5 mg/dL. Urinalysis shows 2+ proteinuria, 2+ leukocyte esterase, and numerous white blood cells, including eosinophils. Peripheral blood smear shows eosinophilia. What is the most likely diagnosis?
✓ Correct Answer: Option B
The sudden rise in creatinine, new rash, eosinophilia in blood and urine, and recent antibiotic use are classic findings for acute interstitial nephritis (AIN). AIN is an inflammatory reaction in the renal interstitium, often drug-induced.