Ultrasound shows polyp on liver. Could it be cancerous? What would cause this? My doctor won't tell me about the important stuff.

Unlikely. Cancer of the liver is unlikely to develop in a 21 year old. But you need to know what this 'polyp" is because that is not the right terminology for a liver lesion. So you need to find ourt what exactly do you have on your liver....It is not a polyp. If you go prepared(read up on internet) and with a list of questions in hand your doctor has to answer you corectly....Alert the nurse re- your con.