It's true that acute mitral regurgitation occurs particularly with Marfans. However, you should have added information about the localisation of the murmur. In this case, with the history of possibly a type A aortic dissection, one could have suspected the presence of acute aortic regurgitation (as a complication of the dissection). However, I would think the answer would remain squatting (increased preload + the increased afterload would favor regurgitation across the aortic valve to the left ventricule).
It's true that acute mitral regurgitation occurs particularly with Marfans. However, you should have added information about the localisation of the murmur. In this case, with the history of possibly a type A aortic dissection, one could have suspected the presence of acute aortic regurgitation (as a complication of the dissection). However, I would think the answer would remain squatting (increased preload + the increased afterload would favor regurgitation across the aortic valve to the left ventricule).