Murmur that is louder with reduced venous return => Hypertrophic cardiomyopathy
HOCM is due to mutations encoding sarcomeres such as myosin binding protein C and beta myosin heavy chain.
FA 19, pg. 305 - May see Mitral regurgitation due to impaired mitral valve closure.
I always find it important to remember that once you get pathology somewhere in the heart, you can expect pathology everywhere behind it, over time.
So in this case you start with HCM -> Mitral regurg -> LA dilation -> A.fib -> LA/LV failure -> Pulm edema -> RHF -> etc.
It's always a matter of time.