List causes for acute aortic regurgitation.
Aortic dissection (ascending aorta)
List causes for chronic aortic regurgitation.
Primary valvular defect
Congenital bicuspid aortic valve: most common cause of AR in young adults in high-income countries
Calcific aortic valve disease: most common cause of AR in older patients in high-income countries 
Rheumatic heart disease: most common cause of AR in lower-income countries 
Connective tissue disorders (e.g., Marfan syndrome, Ehlers-Danlos syndrome)
Aortitis of any etiology (e.g., tertiary syphilis)
Thoracic aortic aneurysm