A 78-year-old man is found collapsed at home. In the Emergency Department, his GCS is 10 (E3 V3 M4), respiratory rate 24/min, heart rate 115/min, BP 90/50 mmHg, temperature 38.5°C, and oxygen saturation 91% on 15L via non-rebreather mask. Chest X-ray shows consolidation in the right lower lobe. What is the most important initial management step within the first hour according to the Sepsis Trust 'Sepsis 6' bundle?