A 28-year-old man comes to the student health clinic because of a 2-day history of sharp chest pain. He describes the pain as "stabbing" and says it is worse when he lies down at night, forcing him to sleep propped up with pillows. During the day, he finds relief by sitting forward at his desk while studying. The pain worsens when he takes a deep breath. One week ago, he had fever, sore throat, and nasal congestion that have since resolved. He takes no medications and has no history of major medical illness. He does not smoke or use illicit drugs. His temperature is 38.1°C (100.6°F), pulse is 92/min, respirations are 16/min, and blood pressure is 118/76 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 98%. Cardiac examination shows a scratchy, high-pitched sound heard best at the left lower sternal border during expiration with the patient leaning forward. The point of maximal impulse is not displaced. There is no chest wall tenderness with palpation. Lungs are clear to auscultation bilaterally. The remainder of the examination shows no abnormalities.
Which of the following is the most likely diagnosis?