Is mitral stenosis a congenital defect or the narrowing of the mitral valve due to scaring?

Scarring: Scarring due to rheumatic heart disease due to infection with certain subtypes of group a beta hemolytic streptococcus.

Can be either: The majority of mitral stenosis is a result of a previous strep infection which cause rheumatic fever. This begins a slowly progressive scarring of heart valves. The aortic and mitral valves most commonly. However there can be congenital mitral stenosis when the mitral valve may not form big enough during fetal development.