A 40-year-old male farmer presents with fever, headache, and diarrhea for 3 days on November 10. Physical exam: temperature 39°C, BP 50/30 mmHg, apathy, facial flushing, conjunctival congestion and edema, and extensive petechiae in both axillae. No hepatosplenomegaly. Lab results: WBC 32×10⁹/L, Hb 160g/L, platelets 34×10⁹/L, urinary protein (+++), stool microscopy shows WBC 5-8/HPF. Which of the following is the most critical and urgent intervention?