A 54-year-old man presents to the ED with sharp pleuritic central chest pain that worsens when he lays supine. It started 5 days ago and has been fairly constant. He denies fever, cough, hemoptysis and has no risk factors for pulmonary embolism. Past medical history is unremarkable. He did have a mild flu-like illness 4 weeks ago. Vital signs are normal. Troponin, D-dimer and chest radiograph are normal. ECG shows no ST changes, PR depression or T wave abnormalities. What is the most likely diagnosis?