In winter, a 42-year-old male presented to the emergency department with fever, headache, and low back pain for 3 days, with a temperature of 40-40.6°C. Examination revealed: BP 70/40 mmHg, pulse 110 beats/min, facial flushing, conjunctival congestion and edema, palatal petechiae and reticular congestion, scattered petechiae on the chest and axilla, no abnormalities in heart and lungs, and positive renal percussion tenderness. The most likely diagnosis is: