Would an abdominal/pelvic mri show a large tumour in the colon causing pencil stools and severe constipation with bloating and nausea?

Yes : Highly unlikely in a 20 year old Best diagnosed by colonoscopy

Not necessarily: To interpret an abdominal imaging study for colon cancer, the person needs to have colon preparation as would be done for colonoscopy, before the imaging study. Unless you have a family history of early onset colon cancer, it is highly unlikely that you have one. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, low fat milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex.