American Board of Internal Medicine (ABIM) Certification Practice Exam

When diagnosing primary CNS lymphoma, what method can aid in confirmation?

1. CT imaging of the abdomen

2. Polymerase chain reaction of cerebrospinal fluid

3. MRI of the brain

4. Bone marrow biopsy

The correct answer is: Polymerase chain reaction of cerebrospinal fluid

When diagnosing primary CNS lymphoma, polymerase chain reaction (PCR) of cerebrospinal fluid (CSF) can provide valuable confirmation. This method is particularly useful because it can detect the presence of specific genetic material or DNA sequences associated with lymphoma cells. In cases of CNS lymphoma, malignant lymphoid cells may shed DNA into the CSF, allowing PCR techniques to identify these cells even if they are not easily visible on imaging studies. While MRI of the brain is a critical imaging modality that can help characterize brain lesions and assess their location, size, and potential features typical of lymphoma, it does not provide definitive confirmation of the diagnosis. Rather, MRI findings can suggest lymphoma based on characteristics such as contrast enhancement and edema, but they cannot establish the diagnosis alone. CT imaging of the abdomen primarily helps evaluate for systemic disease or other causes of symptoms, but it is not useful for confirming primary CNS lymphoma. Similarly, a bone marrow biopsy is not directly relevant in the diagnosis of CNS lymphoma, as this procedure is more appropriate for assessing hematologic conditions where bone marrow involvement is suspected. Thus, the utilization of PCR is a powerful tool in confirming the diagnosis of primary CNS lymphoma by identifying the presence of lymphoma-specific genetic material directly in the cerebrospinal fluid.