USMLE-Rx Step 1 Practice Q's

USMLE-Rx Step 1 Qmax Challenge #1099

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USMLE-Rx Step 1 Qmax Challenge #1099A homeless 37-year-old woman with HIV infection comes to the clinic with a 4-week history of worsening hemiparesis, visual field deficits, and cognitive impairment. The patient’s CD4+ count is 22/mm³. A lumbar puncture shows a normal opening pressure, and cerebrospinal fluid analysis is largely normal. An MRI is performed and is shown in the image.

Which of the following entities is most likely responsible for this patient’s clinical picture?

A. Cortical tuberculoma
B. Cytomegalovirus encephalitis
C. JC virus
D. Primary central nervous system lymphoma
E. Toxoplasmosis


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9 replies »

  1. The correct answer is C. The clinical picture and imaging are consistent with progressive multifocal leukoencephalopathy (PML) secondary to reactivation of latent JC virus infection, which can occur with CD4+ counts <200/mm³. It typically presents with rapidly progressive focal neurologic deficits without signs of increased intracranial pressure. Ataxia, aphasia, and cranial nerve deficits also may occur. Lumbar puncture is nondiagnostic and frequently demonstrates mild elevations in protein and WBCs. Cerebrospinal fluid (CSF) analysis is often normal, though can reveal the presence of myelin basic protein, which is due to demyelination caused by the JC virus. PML typically presents as multiple nonenhancing T2-hyperintense lesions, as shown by arrows in the image. When it PML is suspected, stereotactic biopsy is required for definitive diagnosis, but a positive CSF polymerase chain reaction for JC virus is diagnostic in the appropriate clinical setting. Histology of the lesions shows nuclear inclusions in oligodendrocytes. Although there is no definitive treatment, clearance of JC virus DNA can be observed with response to highly active antiretroviral therapy.

    A is not correct. Uncommon in the developed world, but presenting with increased risk in homeless and HIV patients, cortical tuberculomas are caseating foci within the cortical parenchyma occurring from previous hematogenous mycobacterial bacillemia. The clinical presentation may be similar to that of the current patient; however, presentation would include enhancing nodular lesions on imaging and elevated protein and low glucose on CSF examination.

    B is not correct. Cytomegalovirus (CMV) encephalitis can mimic the appearance of PML, but would be associated with enhancing periventricular white matter lesions in cortical and subependymal regions. CMV encephalitis also is associated typically with more systemic signs and symptoms. Polymerase chain reaction analysis of CSF would be positive for CMV, and histologic exam shows giant cells with eosinophilic inclusions in both the cytoplasm and the nucleus.

    D is not correct. Central nervous system lymphoma typically affects those with CD4+ cell counts <50/mm³. MRI will demonstrate one or more enhancing lesions (50% are multiple and 50% are single) that typically are surrounded by edema, and can produce a mass effect. Central nervous system lymphoma can present with polymerase chain reaction findings positive for Epstein-Barr virus on CSF.

    E is not correct. Space-occupying lesions due to toxoplasmosis infection represent the most common cause of cerebral mass lesions in HIV-infected patients, and typically present with multiple enhancing lesions on MRI. The lesions typically are located at the corticomedullary junction and are surrounded by edema that frequently produces a mass effect and distinguishes its appearance from PML. Positive Toxoplasma serologies can assist in diagnosis, and clinical improvements will result from treatment with sulfadiazine/pyrimethamine or trimethoprim/sulfamethoxazole.


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