Question 1

A patient with HIV infection and a CD4+ T-cell count below 50 cells/µL develops a progressive neurological illness characterized by confusion, motor deficits, and seizures. Brain MRI reveals multiple white matter lesions without mass effect. PCR of the CSF is positive for JC virus. Thepathogenesis of this condition, progressive multifocal leukoencephalopathy (PML), involves:

Options :

A : Direct cytopathic effect of JC virus on neurons.

B : Lytic infection of oligodendrocytes by JC virus, leading to demyelination.

C : Inflammatory demyelination triggered by JC virus infection of astrocytes.

D : Opportunistic infection by JC virus due to impaired cell-mediated immunity.

Answer: B

Question 2

A clinical microbiology laboratory is using flow cytometry to analyze lymphocyte subsets in a patient with suspected immunodeficiency. Which of the following cell surface markers is typically used to identify helper T cells?

Options :

A : CD8

B : CD4

C : CD19

D : CD56

Answer: B

Question 3

A clinical trial is evaluating the efficacy of a new antifungal agent against invasive aspergillosis. Serum galactomannan levels are being used as a biomarker to monitor the infection. A decrease in galactomannan levels during treatment would indicate:

Options :

A : Development of resistance to the antifungal agent.

B : Successful clearance or reduction of the Aspergillus burden.

C : A false-positive result in the galactomannan assay.

D : Concurrent infection with a non-Aspergillus mold.

Answer: B

Question 4

A clinical laboratory performs a cefoxitin disk diffusion test on a Staphylococcus aureus isolate from a wound culture. The zone of inhibition measures 18 mm. According to current CLSI M100 guidelines, how should this result be interpreted regarding oxacillin/methicillin susceptibility?

Options :

A : Susceptible (report oxacillin as susceptible)

B : Resistant (report oxacillin as resistant; infer MRSA)

C : Intermediate (requires MIC testing)

D : Indeterminate (requires mecA gene testing)

Answer: B

Question 5

A clinical microbiology laboratory is investigating a cluster of Clostridium difficile infections in a hospital. Which of the following methods is MOST appropriate for determining if the isolates from different patients are related?

Options :

A : Gram staining and morphology

B : Antimicrobial susceptibility testing

C : Enzyme immunoassay for C. difficile toxins

D : Molecular typing (e.g., PCR ribotyping)

Answer: D

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