Mr. Yien reports to the clinic after having passed reddish- colored urine this morning. He reports that he has been feeling very fatigued for several weeks and hasn't been urinating as much as usual. He also tells you that three years ago he had a kidney transplant. You order a urinalysis, which is positive for large amounts of blood and protein. What is potentially causing Mr. Yien's problems, given his medical history? Explain.