WorksheetsClinical Microbiology and Molecular Diagnostics Worksheet
Total questions: 13
Worksheet time: 7mins
A 67-year-old man with diabetes is admitted with fever, rigors, and hypotension. You suspect bacteremia secondary to a urinary tract infection. The ED resident plans to draw blood cultures from a peripheral vein after starting empiric piperacillin–tazobactam because “we don’t want to delay antibiotics.” Which is the best advice you should give as the clinical microbiologist?
Draw one blood culture set now from a peripheral vein after antibiotics.
Draw one blood culture set now from the existing peripheral IV line before antibiotics.
Draw at least two sets of blood cultures from different venipuncture sites before starting antibiotics.
Draw one set from a central line and one set from a peripheral line after antibiotics.
Give antibiotics first, then draw blood cultures within 4 hours.
A 25-year-old woman presents with acute onset fever, headache, neck stiffness, and photophobia. Lumbar puncture is performed. The house officer wants to send the CSF in one tube to the microbiology laboratory “for everything.” As the clinical microbiology resident, which is the most appropriate instruction for CSF handling for suspected acute bacterial meningitis?
Send a single CSF tube to microbiology; other tests can be done on serum.
Collect multiple CSF tubes, second tube for microbiology testing.
Refrigerate CSF immediately to preserve bacterial viability.
Add formalin to the CSF to stabilize cells for Gram stain.
Centrifuge CSF in the ward and send only the sediment.
A 30-year-old man with chronic cough, weight loss, and night sweats has a sputum sample tested using a cartridge-based nucleic acid amplification test (NAAT) for Mycobacterium tuberculosis. The result is “MTB not detected” with no rifampicin resistance. The smear is negative for AFB. However, after 4 weeks, the culture on Löwenstein-Jensen medium grows M. tuberculosis complex. Which is the most appropriate explanation for this discordant result?
NAAT is less specific than culture and commonly gives false negatives.
Low bacillary load in the specimen can lead to false-negative NAAT with subsequent positive culture.
The culture result is likely a contamination by non-tuberculous mycobacteria.
NAAT detects only rifampicin-resistant strains of M. tuberculosis.
The initial NAAT result must have been misreported; this pattern is impossible.
A 42-year-old woman with chronic lymphadenopathy and low-grade fever undergoes excisional biopsy of a cervical lymph node. Histopathology shows granulomatous inflammation with caseous necrosis. AFB stain is negative. The clinician asks which microbiologic test would most appropriately increase diagnostic yield for TB on the same tissue.
Gram stain and routine bacterial culture on the tissue specimen
Fungal stain and culture on the tissue specimen
Mycobacterial culture plus NAAT for M. tuberculosis on the tissue specimen
Serologic antibody test for M. tuberculosis
Interferon-gamma release assay on lymph node tissue
A 60-year-old man presents with septic shock due to pneumonia. Initial lactate is 5.2 mmol/L. Two sets of blood cultures are drawn and broad-spectrum IV antibiotics are started. After 48 hours, all blood cultures remain negative, but the patient is still febrile and hypotensive. Which is the most appropriate interpretation/next step?
Negative blood cultures exclude sepsis; narrow antibiotics to oral therapy and star antifungal because very likely the cause is fungal infection
Continue to manage as sepsis despite negative cultures; consider non-bacterial causes and repeat cultures if indicated.
Stop all antibiotics and repeat blood cultures to avoid masking the pathogen.
Assume a viral cause and start antiviral therapy. Stop the antibiotic immediately
Request the lab to incubate the cultures for 14 days for all patients with sepsis.
A 70-year-old woman with an indwelling central venous catheter develops fever and chills. Two sets of blood cultures are taken: one from the central line and one from a peripheral vein. Both sets grow coagulase-negative staphylococci, but the central line culture becomes positive at 10 hours, and the peripheral culture at 26 hours. Which is the best interpretation?
Likely contamination from skin flora; no treatment needed.
True catheter-related bloodstream infection, given differential time to positivity.
Lab error; discard both results.
Peripheral line is the source of infection.
Contamination is confirmed because both sets are positive.
A 58-year-old man undergoes elective colon surgery. On postoperative day 6, he develops fever and purulent discharge from the surgical wound; culture grows ESBL-producing Escherichia coli. You are asked to advise the surgical team about interpretation. Which is the most accurate statement regarding this case?
This represents a superficial incisional SSI likely due to skin flora.
The timing and organism are consistent with a deep or organ/space SSI.
The infection is hospital-acquired but not considered an SSI because it occurs after 48 hours.
ESBL-producing organisms cannot cause SSI; contamination is more likely.
This is most likely a community-acquired infection unrelated to surgery.
Your hospital plans to implement MRSA screening for ICU patients. A sample from an ICU patient shows growth of S. aureus on chromogenic MRSA screening agar, but the cefoxitin disk diffusion test on Mueller–Hinton agar shows a clear susceptible zone, and mecA PCR is negative. Which is the best conclusion?
This isolate should be reported as MRSA based only on chromogenic agar.
This represents a heterogeneous MRSA; report as MRSA regardless of cefoxitin.
The chromogenic agar result is likely a false positive; report as MSSA based on cefoxitin and mecA results.
mecA PCR is unreliable; use oxacillin disk only.
Report the isolate as vancomycin-intermediate S. aureus (VISA).
A 65-year-old man in the ICU with multiple abdominal surgeries develops persistent fever despite broad-spectrum antibiotics. Blood cultures grow Candida glabrata in two bottles. The patient has been receiving fluconazole prophylaxis. Which is the most appropriate initial antifungal therapy?
High-dose oral fluconazole
Oral itraconazole
An echinocandin (e.g., caspofungin, micafungin)
Amphotericin B lipid formulation only if fluconazole fails
Topical nystatin
An 80-year-old woman with an indwelling Foley catheter in a nursing home has asymptomatic candiduria with Candida albicans repeatedly isolated at ≥10^5 CFU/mL. She is afebrile with no urinary symptoms. The catheter has been in place for 2 months. What is the best management?
Start oral fluconazole for 14 days.
Remove/replace the catheter and no antifungal therapy if she remains asymptomatic.
Start IV amphotericin B.
Treat only if blood cultures become positive.
Begin bladder irrigation with amphotericin B.
A 33-year-old man who recently returned from a 6-month stay in a rural area with caves and bat colonies presents with fever, weight loss, and diffuse pulmonary infiltrates. BAL culture later grows a mold that at 25°C produces tuberculate macroconidia, while at 37°C it grows as a yeast. Which of the following tests would best support early diagnosis before culture results are available?
Serum antibody test for Cryptococcus neoformans
Urine antigen detection for Histoplasma capsulatum
Serum galactomannan assay for Aspergillus fumigatus
Serum cryptococcal antigen
KOH preparation of skin scrapings
A 35-year-old healthcare worker in an intensive care unit in South Asia develops fever, headache, and rapidly progressive encephalitis after caring for several patients with severe respiratory illness. An outbreak of Nipah virus has been reported in the region. You are asked to advise on infection-control measures while laboratory confirmation is pending. Which is the most appropriate recommendation?
Standard precautions only; Nipah virus is not transmitted person-to-person.
Standard + contact + droplet precautions; consider airborne precautions during aerosol-generating procedures.
Airborne precautions only.
Droplet precautions only.
Contact precautions only.
A 45-year-old surgeon undergoes routine screening. His serology results are: • HBsAg: negative • Anti-HBs: positive (high titer) • Anti-HBc total: negative. Which is the most accurate interpretation?
Past natural infection with HBV, now immune.
Window period of acute HBV infection.
Chronic HBV infection with low viral load.
Immunity due to successful hepatitis B vaccination.
False-positive anti-HBs; repeat all tests.
