MLT GlobeDHA exam preparation (Dubai) – page 30
700 practice MCQs for the DHA medical laboratory exam. Level: Basic to intermediate.
Most members of the Bacteroides fragilis group are resistant to penicillin mainly because they:
B. fragilis group organisms produce chromosomal beta-lactamases that break down penicillin and many cephalosporins. Metronidazole and beta-lactam/beta-lactamase inhibitor combinations remain active against most isolates.
An intravenous catheter tip is rolled across a blood agar plate (semiquantitative method). Which result suggests catheter-related infection?
In the roll-plate (Maki) method, 15 or more colonies suggest the catheter is colonised and a likely source of infection. Fewer colonies usually reflect contamination.
During verification of a new AST system, an isolate is reported 'susceptible' by the new system but is 'resistant' by the reference method. This is classified as:
A false-susceptible result is a very major error because it may lead to ineffective therapy. A false-resistant result is a major error; a minor error involves the intermediate category.
An Enterobacter cloacae isolate tests susceptible to ceftriaxone. CLSI advises that resistance may develop during therapy with third-generation cephalosporins because this organism:
Enterobacter cloacae has a chromosomal AmpC that can be induced or selected (derepressed mutants) during therapy. PBP2a is the staphylococcal mecA product.
Blood culture bottles are collected at night, and loading onto the continuous monitoring system will be delayed. How should they be held?
Bottles should stay at room temperature; refrigeration can kill fastidious organisms. Long pre-incubation outside the instrument can also cause missed positives, so delay should be minimised.
Which member of the Enterobacterales is intrinsically resistant to colistin?
Serratia marcescens, along with Proteus, Providencia and Morganella, is intrinsically resistant to polymyxins. Colistin resistance in E. coli or K. pneumoniae is acquired, for example by mcr genes.
Small colonies grow on bile esculin agar with blackening. The isolate is catalase positive, and the Gram stain shows short gram-positive rods. The most likely organism is:
Listeria hydrolyzes esculin in the presence of bile, like enterococci, but it is a catalase-positive rod. Enterococci are catalase-negative cocci, and group B streptococci are esculin negative.
The gene for diphtheria toxin (tox) in Corynebacterium diphtheriae is carried on:
Only strains lysogenized by a beta-corynephage carrying the tox gene produce diphtheria toxin. This is why toxin production must be tested, for example by Elek test or PCR.
CLSI does not accept vancomycin disk diffusion for Staphylococcus aureus because this method:
Zone sizes of VISA isolates overlap with those of susceptible strains, so disk diffusion misses VISA. An MIC method is required for vancomycin testing of S. aureus.
Which result shows high-level gentamicin resistance in Enterococcus faecalis, meaning ampicillin plus gentamicin will not be synergistic?
High-level aminoglycoside resistance is screened with 500 µg/mL gentamicin in broth (or a 120 µg disk). Standard 10 µg disks and low concentrations are not used for enterococci because low-level resistance is intrinsic.
Which test result helps separate Enterococcus faecium from Enterococcus faecalis?
E. faecium ferments arabinose, while E. faecalis does not. PYR, 6.5% NaCl growth and bile esculin are positive for both species and are used to identify the genus.
A 7-year-old with pharyngitis has a negative rapid antigen detection test for group A streptococcus. According to IDSA guidance, what should follow?
Rapid antigen tests are specific but not fully sensitive, so a negative result in children and adolescents should be backed up by culture (or a sensitive molecular test). ASO shows past infection, not current pharyngitis.
A slow-growing acid-fast isolate is inoculated onto a medium containing p-nitrobenzoic acid (PNB). It fails to grow. This supports identification as:
PNB inhibits members of the M. tuberculosis complex but most NTM grow in its presence. This makes PNB a simple way to separate MTBC from NTM.
An interferon-gamma release assay (IGRA) is preferred to the tuberculin skin test in BCG-vaccinated people because IGRA antigens (ESAT-6 and CFP-10):
ESAT-6 and CFP-10 are coded in the RD1 region, which is deleted in all BCG strains, so vaccination does not give a positive IGRA. The tuberculin (PPD) mixture contains antigens shared with BCG.
A man treated with intravesical BCG for bladder cancer develops fever and prostatitis. A slow-growing Mycobacterium tuberculosis complex isolate grows from his urine. The most likely organism is:
BCG is a live attenuated M. bovis strain, and local or disseminated infection can follow bladder instillation. M. kansasii is not a member of the M. tuberculosis complex.
A 20-year-old has his third episode of invasive meningococcal disease. Which laboratory test is most useful to look for an underlying cause?
Deficiency of terminal complement components (C5–C9) greatly increases the risk of recurrent Neisseria infections. CH50 screens the whole classical pathway, including these components.
A child has had tender axillary lymphadenopathy for 3 weeks after scratches from a new kitten. Which laboratory test is most useful to confirm the likely cause?
Cat-scratch disease is caused by Bartonella henselae, which grows very slowly and is rarely cultured. Serology (IgG/IgM) or PCR of node tissue is used for diagnosis.
Which carbapenem has no useful activity against Pseudomonas aeruginosa and should not be reported for it?
P. aeruginosa is intrinsically resistant to ertapenem, mainly through efflux and poor penetration. Meropenem, imipenem and doripenem are active against wild-type strains.
A farmer in a tropical region develops a wound infection and sepsis after contact with stagnant water. The isolate is a gram-negative rod producing violet-black colonies. The most likely organism is:
Chromobacterium violaceum lives in tropical soil and water and produces the violet pigment violacein. Infections are rare but often severe. Serratia marcescens produces red pigment.
When Burkholderia pseudomallei is suspected, culture plates should be kept closed and not sniffed, and work should be done in a biological safety cabinet because:
B. pseudomallei is a Tier 1 select agent and can infect laboratory staff through aerosols. Suspected isolates should be handled in a biosafety cabinet and referred to a reference laboratory.