A spherical tumor 3 cm in diameter consisting of prostate cells may result in a PSA blood level of about 20 nanograms/milliliter of blood. The most common test can only detect the presence of PSA in the blood if it is greater than about 0.1 ng/ml. An MRI can detect a cancerous tumor down to about a diameter of 1 mm. Which is a more sensitive test for the return of prostate cancer, the blood test or an MRI

MRI test.

Explanation:

Cancer is the abnormal growth of the cells  that might occur due to the spontaneous mutation or due to the intake of the carcinogenic substances in the body.

The amount of PSA is directly proportional to the tumor volume. Since, the blood test cannot detect the tumor down to about a diameter of 1 mm. The information given in the question shows that the diameter of the tumor is around 5 mm that can not be detected by teh blood test but can be easily detectable by MRI.

Thus, the answer is MRI test.

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