A 38-year-old woman with severe sepsis secondary to urosepsis presents with acute kidney injury stage 3 (KDIGO criteria). She is hypotensive (MAP 55 mmHg) despite 30 mL/kg crystalloid bolus and has oliguria (urine output 0.3 mL/kg/hour). Blood cultures are pending. After initiation of broad-spectrum antibiotics and vasopressor support (noradrenaline), what is the drug of choice for renal replacement therapy initiation in this patient?