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 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 3

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 4

A clinical laboratory manager is implementing a new automated blood culture system. Which aspect is LEAST likely to be part of the initial verification/validation process before clinical use?

Options :

A : Comparison of time-to-detection with the previous system

B : Verification of detection of a panel of common bloodstream pathogens

C : Assessment of the system's contamination rate over one year of routine use

D : Establishing workflow and training competency for laboratory staff

Answer: C

Question 5

A microbiology laboratory is processing a stool sample for ova and parasites. A large (approximately 100 µm), thick-shelled, oval egg with a flattened side is observed. This egg MOST likely belongs to:

Options :

A : Ascaris lumbricoides

B : Enterobius vermicularis

C : Taenia species

D : Trichuris trichiura

Answer: A