A 3-year-old child presents to the emergency department with a 2-day history of diarrhea and vomiting. On examination, the child is lethargic, has sunken eyes, weak pulses, and a capillary refill time of 4 seconds. Blood pressure is 70/40 mmHg. After establishing IV access and initiating fluid resuscitation with 20 mL/kg bolus of normal saline, the child remains hypotensive and tachycardic. Which investigation is most appropriate to guide further management and identify the underlying cause of shock?