A 60-year-old woman with a history of mild ulcerative colitis previously well controlled on mesalamine as well as metastatic breast cancer on Palbociclib and anastrozole presents with worsening diarrhea. Stool is intermittently bloody, and she has been having up to 7 loose stools per day. C. Difficile test, stool bacterial PCR, and stool parasite panel were negative. Fecal calprotectin was 144 ug/g. A colonoscopy demonstrates colitis to 35 cm from the anal verge that is scored as Mayo 2. In addition to assessing disease activity, biopsies should be obtained to assess for the presence of which of the following opportunistic infections?