This patient likely has colitis resulting from cytomegalovirus (CMV) infection. CMV infection is a common complication of transplantation, especially in the first few months after transplantation when immunosuppression is typically highest. Unless donor and recipient are CMV seronegative and the risk is low, prophylaxis with valganciclovir is often used during this time period. This patient recently finished the prophylaxis period and is at risk for CMV reactivation. She has symptoms typical for colitis; the colon and esophagus are common sites for CMV disease after transplantation. Hepatitis, gastritis, and small bowel enteritis may also occur, although less often. Quantitative polymerase chain reaction testing in serum for CMV can be suggestive, but colonoscopy with biopsy for characteristic histopathology and viral tissue culture provide a definitive diagnosis.

Candidal infection does not cause colitis, even in immunocompromised patients. Candida species are part of the normal bowel flora, with positive stool cultures commonly occurring in the absence of systemic disease. This patient has no evidence of invasive candidal infection, which makes this an unlikely cause of her presenting symptoms.

Although she is at risk for Clostridium difficile colitis, her negative results on nucleic acid testing makes this a less likely cause of her clinical findings than CMV disease.

Although mycophenolate can cause diarrhea as an adverse effect, this patient is at significant risk for opportunistic infections, which must be considered more likely, especially in the presence of fever.

Polyoma BK virus is a DNA virus present in approximately 5% of kidney transplant recipients. Its activity is usually related to the degree of immunosuppression and can occur at this stage or later after transplantation. However, it primarily causes tubulointerstitial nephropathy, not colitis.