A 28-year-old man presents with acute onset sharp chest pain relieved by sitting forward and worsened by lying flat and deep inspiration. On examination, a scratchy pericardial friction rub is heard at the left lower sternal border. His ECG shows diffuse concave-up ST elevation across multiple territories. The finding marked **B** in the diagram is noted prominently in lead II. Which of the following best explains the pathophysiology of the finding marked **B**?