A 58-year-old man with diabetes mellitus type 2 presents to the emergency department with a 2-day history of fever (39.5°C), chills, and dysuria. He has a history of recurrent urinary tract infections. On examination, he is tachycardic (HR 118/min), tachypneic (RR 24/min), hypotensive (BP 88/54 mmHg), and has costovertebral angle tenderness. Laboratory investigations show: WBC 18,500/μL, lactate 4.2 mmol/L (normal <2), creatinine 2.1 mg/dL (baseline 1.0), and blood cultures are pending. Urine culture shows gram-negative rods. Which of the following is the most appropriate immediate management step?