A 42-year-old woman with a history of rheumatoid arthritis (on methotrexate) presents to the emergency department with a 3-day history of fever, cough, and dyspnea. On examination: temperature 39.8°C, BP 92/58 mmHg, heart rate 128/min, respiratory rate 28/min, oxygen saturation 88% on room air. Lung auscultation reveals bilateral crackles. Blood cultures are pending. Lactate is 4.2 mmol/L (normal <2). Chest X-ray shows bilateral infiltrates. The patient is started on broad-spectrum antibiotics and fluid resuscitation. Despite 2 liters of normal saline, the BP remains 90/55 mmHg, urine output is 0.3 mL/kg/hr, and extremities remain warm. Which type of shock is this patient experiencing, and what is the underlying pathophysiologic mechanism?