A 38-year-old man with acute bacterial peritonitis presents to the emergency department with hypotension (BP 85/50 mmHg), tachycardia (HR 128/min), and altered mental status. Blood cultures are drawn and broad-spectrum antibiotics are initiated. After fluid resuscitation with 30 mL/kg crystalloid over 3 hours, his BP remains 92/55 mmHg with persistent tachycardia and oliguria. Which is the drug of choice for hemodynamic support in this patient?