How can a 25 year old male in good condition get an artery blockage?

I went to the hospital for angina a few months ago and it turned out it was just pleurisy. Or so I thought. My doc gave me a stress test just to make sure I was okay and it was abnormal. He referred me to a cardiologist who gave me a whole slew of tests, all of which were normal except my cardiac MRI. Apparently there's a blockage of some sort. I'm a 25 year old male and in relatively good condition. I eat right, I exercise, I'm in the military. How on earth can I have a blockage? As far as my symptoms go I still get a little tightness in my chest every now and then for no reason, but nothing serious and I have noticed trouble when I run. But that's it.