A 31-year-old man experienced chest pain, became increasingly dyspneic and nauseated, and lost consciousness multiple times. Seven days after the appearance of these symptoms, he was found dead in his sleep. External examination of the body by the medical examiner shows no evidence of trauma. The body is 166 cm (5 ft 5 in) in height and weighs 75 kg (BMI 27). The gross appearance of the chest cavity at autopsy with the pericardial sac opened is shown in the figure. What is the most likely underlying cause of his death?