A 3-year-old child from rural India presents with a 2-day history of acute diarrhea and vomiting. On examination, the child is lethargic, with sunken eyes, weak radial pulse, capillary refill time >2 seconds, and systolic BP 70 mmHg. Urine output is <0.5 mL/kg/hr. 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 refractory shock?