A 56-year-old man develops sharp central chest pain 36 hours after an uncomplicated inferior myocardial infarction. The pain is worse when he lies flat or takes a deep breath and eases when he sits forward. A scratchy sound is heard with each heartbeat. The ECG shows widespread concave ST elevation rather than new changes confined to the infarct territory, and serial troponin concentrations are unchanged. Which one of the following is the most likely diagnosis?