A 20-year-old man comes to the physician for routine examination. He is afebrile and has clear breath sounds throughout. Cardiac examination shows a mid to late diastolic murmur on auscultation, loudest at the cardiac apex. 
Which of the following is the most likely cause for this patient’s murmur?
Acute pericarditis
Congenital heart defect
Infective endocarditis
Rheumatic fever