A 48-hour-old preterm neonate (32 weeks gestation) born to a mother with chorioamnionitis presents with tachypnea (respiratory rate 65/min), poor perfusion, and a capillary refill time of 3 seconds. Blood culture, urine culture, and CSF culture have been obtained. The neonate is on supplemental oxygen. Investigations show: WBC 3,200/μL, platelets 95,000/μL, CRP 18 mg/L. What is the most appropriate immediate next step in management?