A 22-year-old male presented on August 10 with half a day of diarrhea (20+ episodes of watery stools), 5-6 episodes of vomiting, no fever or abdominal pain, oliguria, normal body temperature, BP 70/50 mmHg (10.8/6.7 kPa), sunken eyes, and dry mouth/tongue. Laboratory tests: Hb 160g/L, WBC 10.1×10⁹/L. The most critical treatment is: