A 3-year-old child presents to the emergency department with severe dehydration and hypotension (systolic BP 60 mmHg) following 3 days of acute gastroenteritis with profuse diarrhea. After rapid assessment, 20 mL/kg bolus of normal saline is initiated. Despite fluid resuscitation, the child remains hypotensive with poor perfusion, weak pulses, and altered mental status. What is the drug of choice for inotropic support in this case of refractory hypovolemic shock?