A 28-year-old male presents with precordial pain radiating to the left shoulder, exacerbated on inspiration and relieved by sitting upright, accompanied by fever, chills, BP 110/80 mmHg, heart rate 110/min (regular rhythm), and temperature 38.5°C. Other examinations are unremarkable. ECG shows ST-segment elevation (except in aVR). On day 3 of hospitalization, the patient develops hypotension, elevated venous pressure, distended neck veins, and shock. The most likely diagnosis is: