A 42-year-old woman with severe sepsis secondary to urosepsis is admitted to the ICU. She develops acute kidney injury with serum creatinine 3.8 mg/dL, oliguria (urine output 150 mL/24 h), and muddy brown casts on urinalysis. FENa is 3.2%. She is hypotensive (BP 88/52 mmHg) despite 2 L of crystalloid resuscitation. Blood cultures are pending. What is the first-line pharmacological agent to improve renal perfusion in sepsis-induced acute tubular necrosis?