A 6-month-old infant is brought to the emergency department with a 3-day history of fever, poor feeding, and lethargy. Vital signs show: heart rate 160/min, respiratory rate 52/min, blood pressure 65/35 mmHg, and capillary refill time of 3 seconds. The child has a petechial rash on the trunk and limbs. After initiating IV access and fluid resuscitation, the clinical team suspects meningococcal sepsis. Which investigation is most appropriate to confirm the diagnosis and guide antibiotic selection?