Question 1

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 2

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 3

During the investigation of a foodborne illness outbreak linked to raw sprouts, several bacterial isolates are obtained from patient stool samples and the implicated food source. Which of the following molecular methods would be MOST useful for establishing a definitive link between the clinical isolates and the food isolate?

Options :

A : Gram staining and biochemical profiling

B : Serotyping based on O and H antigens

C : Pulsed-field gel electrophoresis (PFGE)

D : Antimicrobial susceptibility testing

Answer: C

Question 4

A clinical isolate of Klebsiella pneumoniae from a patient with a refractory urinary tract infection exhibits resistance to carbapenems (imipenem, meropenem, ertapenem). PCR analysis reveals the presence of the blaKPC gene. Which of the following enzymatic mechanisms is responsible for this resistance?

Options :

A : Hydrolysis of the beta-lactam ring, leading to an inactive antibiotic derivative.

B : Modification of the ribosomal target site, preventing antibiotic binding.

C : Efflux of the carbapenem antibiotic out of the bacterial cell.

D : Trapping of the carbapenem molecule by a decoy protein.

Answer: A

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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