A 5-year-old child with community-acquired pneumonia and sepsis presents with signs of septic shock: fever (39.5°C), tachycardia (heart rate 140/min), tachypnea (respiratory rate 45/min), delayed capillary refill (>2 seconds), and hypotension (systolic BP 70 mmHg). After rapid assessment and initiation of 30 mL/kg crystalloid bolus over 15 minutes, the child remains hypotensive and poorly perfused. What is the drug of choice for vasopressor support in this case of refractory septic shock?