Differential Diagnosis
- Single most likely diagnosis
- Immune-mediated checkpoint inhibitor colitis: This is the most likely diagnosis given the patient's history of being on a nivolumab immunotherapy clinical trial and the presentation of fever, abdominal pain, nausea, diarrhea, and colonoscopy findings of diffuse ulcerations, erythema, and loss of vascularity, along with an apoptotic colopathy pattern on biopsies. The negative tests for C. difficile, other bacterial and protozoan pathogens, and CMV further support this diagnosis.
- Other Likely diagnoses
- Inflammatory bowel disease (IBD): Although less likely given the acute onset in the context of immunotherapy, IBD could present with similar symptoms and findings. However, the apoptotic colopathy pattern is more suggestive of immune-mediated damage rather than the chronic inflammation seen in IBD.
- Do Not Miss diagnoses
- CMV colitis: Despite the negative CMV test, it's crucial not to miss this diagnosis due to its potential severity and the fact that a single negative test does not entirely rule out the infection, especially in immunocompromised patients. Repeat testing or consideration of empirical treatment might be necessary if clinical suspicion remains high.
- C. difficile colitis: Although the initial test was negative, C. difficile infection can be notoriously difficult to diagnose, and a single negative test does not rule out the infection, particularly if the patient has been on antibiotics or has other risk factors.
- Rare diagnoses
- Other infectious colitides (e.g., due to bacteria, viruses, or parasites not tested for): These could present similarly but are less likely given the negative initial panels.
- Ischemic colitis: Although rare, this could be considered if there were signs of vascular compromise or if the patient had risk factors for ischemic events. However, the biopsy findings and clinical context make this less likely.
- Radiation colitis: Given the patient's history of rectal cancer and potential for radiation therapy, this could be a consideration, but the description and biopsy findings are more consistent with an immune-mediated process.