MLT GlobeSCFHS exam preparation (Saudi Arabia) – page 32
749 practice MCQs for the SCFHS medical laboratory exam. Level: Intermediate.
A kidney transplant recipient on strong immunosuppression has slowly rising creatinine. Which laboratory test is used to screen for polyomavirus nephropathy?
BK polyomavirus reactivates under immunosuppression and can damage the graft; quantitative plasma BK DNA PCR is used for screening and monitoring. Serology is not useful because most adults are already seropositive.
In disk diffusion testing, plates should go into the incubator within 15 minutes after the disks are applied. If they stand for an hour at room temperature, the zones will most likely be:
Drug keeps diffusing at room temperature while bacterial growth is delayed, so the zone boundary forms farther out. Inactivation of the drug is not the main effect.
For Staphylococcus aureus, CLSI recommends the cefoxitin disk rather than the oxacillin disk for disk diffusion because cefoxitin:
Cefoxitin strongly induces mecA/PBP2a expression and gives more reliable, easier-to-read results; a resistant cefoxitin result is reported as oxacillin resistant. It is a surrogate, not a treatment choice for MRSA.
In the Carba NP test, a carbapenemase-producing isolate changes the phenol red indicator from red to yellow because:
Hydrolysis of the beta-lactam ring of imipenem produces a carboxylic acid, which lowers the pH and turns phenol red yellow. Ammonia would raise the pH, not lower it.
A nasopharyngeal specimen for culture of Bordetella pertussis should be inoculated onto:
Regan-Lowe (charcoal-horse blood with cephalexin) neutralises inhibitory fatty acids and supports B. pertussis. Cystine-tellurite agar is used for Corynebacterium diphtheriae.
Blood culture bottles are designed so that blood is diluted in broth at a ratio of about:
A 1:5 to 1:10 dilution reduces the effect of natural bactericidal factors and antibiotics in blood while keeping enough blood for detection.
An Enterococcus faecalis from urine shows a large zone around a cefazolin disk. How should cephalosporins be reported?
Enterococci have low-affinity penicillin-binding proteins and are intrinsically resistant to cephalosporins, which can appear active in vitro. Reporting them as susceptible could lead to treatment failure.
For prenatal screening of group B streptococcus colonisation, the recommended specimen and processing is:
Swabbing both the lower vagina and rectum and using selective enrichment broth (e.g. LIM broth) gives the highest recovery. Cervical swabs and MacConkey agar are not suitable.
Which Shigella species is ornithine decarboxylase positive and a late lactose fermenter (ONPG positive)?
S. sonnei (serogroup D) is typically ODC positive and ONPG positive, fermenting lactose slowly. The other species are usually ODC negative.
Which TSI reaction is typical of Salmonella Typhi after 18–24 hours?
S. Typhi ferments glucose without gas and makes only a small wisp of H2S near the slant–butt junction. Most other Salmonella serotypes produce gas and heavy H2S.
In a typhoid-endemic area, a patient with fever has a single Widal test with an O agglutinin titre of 1:80. The best interpretation is:
A single low Widal titre is unreliable because of background antibodies, vaccination and cross-reactions. A fourfold rise between acute and convalescent sera, or culture, is needed.
In the CLSI ESBL confirmatory disk test for an E. coli isolate, the ceftazidime zone is 14 mm and the ceftazidime-clavulanate zone is 22 mm. The result indicates:
An increase of ≥5 mm in zone size with clavulanate compared with the cephalosporin alone confirms ESBL production. Here the increase is 8 mm. AmpC enzymes are not inhibited by clavulanate.
A wound culture has Proteus swarming over the whole blood agar plate, hiding a possible Staphylococcus aureus. Which medium best helps recover the gram-positive coccus?
Phenylethyl alcohol inhibits gram-negative rods and prevents Proteus swarming, allowing gram-positive cocci to grow as isolated colonies. MacConkey and Hektoen inhibit gram-positive organisms.
Which is the correct way to write the organism commonly called 'Salmonella Typhimurium'?
Most human Salmonella belong to S. enterica subsp. enterica; serovar names are not species, so they are capitalized and not italicized, often shortened to Salmonella Typhimurium.
A lactose-negative, H2S-positive stool isolate must be screened as possible Salmonella rather than Proteus. Which result supports Salmonella?
Salmonella is urease and PAD negative, while Proteus is urease and PAD positive and swarms. Both genera produce H2S, so H2S does not separate them.
A catalase-negative, non-motile gram-positive rod in long chains grows from the blood of a patient with a central line. It has a very high vancomycin MIC. The most likely genus is:
Lactobacillus is catalase negative, and many clinical species (such as L. rhamnosus) are intrinsically vancomycin resistant. Corynebacterium, Listeria and Bacillus are catalase positive.
A bronchoalveolar lavage is sent with a request to rule out nocardiosis. How long should routine culture plates be kept?
Nocardia grows slowly and may take a week or longer to form chalky colonies, so plates should be kept for up to 2–3 weeks. Selective media such as BCYE also help.
A Streptococcus pneumoniae isolate from sputum gives a 14 mm zone around a 1 µg oxacillin disk. What should the laboratory do next?
An oxacillin zone ≥20 mm predicts penicillin susceptibility. A zone ≤19 mm does not prove resistance, because it can occur with susceptible or resistant strains, so a penicillin MIC must be performed.
A healthy nurse has a positive interferon-gamma release assay at pre-employment screening. She has no symptoms and a normal chest X-ray. The best interpretation is:
IGRA shows immune sensitisation to M. tuberculosis but does not separate latent infection from active disease; clinical and radiological findings suggest latent infection. BCG does not cause a positive IGRA.
A gastric aspirate from a child with suspected tuberculosis cannot be processed within 4 hours. The correct action is to:
Gastric acid rapidly kills mycobacteria, so a delayed specimen is neutralised with sodium carbonate. Formalin would kill the organisms and prevent culture.