A lot of interesting comments. It is not just about your academic interests, it is about your passions. If you are fascinated by buildings and bridges, then architecture or Civil Engineering, if you are interested in all areas of physics, Mechanical engineering is a study of applied physics. I do product development because I like creating new products. I am in medical because I like making products that have a positive impact on peoples lives. But Mechanical engineers have a lot of choices in career paths.
Find you passion, follow the education path that allows you to work in that field, and you will enjoy your work life.