A 52-year-old man comes to the clinic for decreased urine output and malaise for 3 days. Two weeks ago, he was treated with an antibiotic for cellulitis. Since then, he has had low-grade fevers and an itchy, diffuse rash. He has no history of chronic kidney disease. Medications include the antibiotic, lisinopril, and atorvastatin. Temperature is 38.1°C (100.6°F), blood pressure is 138/84 mmHg. Physical examination shows a morbilliform rash on the trunk and mild bilateral flank tenderness. Laboratory studies show:

• Serum creatinine: 3.0 mg/dL (baseline 0.9 mg/dL)
• BUN: 42 mg/dL
• Urinalysis: 1+ protein, 2+ blood
• Urine microscopy: numerous WBCs; no casts; no bacteria

Which of the following is the most appropriate next step in management to prevent further kidney injury?