MLT GlobeQCHP exam preparation (Qatar) – page 24
749 practice MCQs for the QCHP medical laboratory exam. Level: Intermediate.
Which HLA typing method gives the highest (allele-level) resolution?
Sequencing-based typing, especially next-generation sequencing, defines HLA alleles at high resolution. Serologic typing only defines broad antigen groups.
After a severe sore throat, a 19-year-old develops thrombophlebitis of the internal jugular vein and multiple lung abscesses. Anaerobic blood culture bottles grow pleomorphic gram-negative rods. The most likely organism is:
This is Lemierre syndrome, classically caused by F. necrophorum spreading from the pharynx to the jugular vein with septic emboli. Veillonella is a gram-negative coccus, and Clostridium is gram positive.
A diarrhoeal stool gives a positive glutamate dehydrogenase (GDH) antigen and a negative toxin A/B immunoassay for Clostridioides difficile. The next step in a multistep algorithm is:
Discordant GDH-positive, toxin-negative results are resolved with NAAT for the toxin gene. GDH alone does not separate toxigenic from non-toxigenic strains, and repeating the insensitive toxin EIA adds little.
A woman with a long-term intrauterine device has a pelvic abscess. Pus contains yellow granules; the Gram stain shows branching gram-positive rods, and anaerobic colonies look like molar teeth. The organism is most likely:
Actinomyces israelii forms sulfur granules and molar-tooth colonies, and causes pelvic actinomycosis with IUDs. Nocardia is aerobic and partially acid-fast.
An anaerobic jar is checked 2 hours after setup and the resazurin indicator is still pink. What does this mean?
Resazurin is colourless when reduced (anaerobic) and pink when oxidized. A pink indicator shows oxygen is present, often from a leaking lid, a failed gas generator or an inactive catalyst.
One of four blood culture bottles grows a gram-positive diphtheroid-like rod in the anaerobic bottle after 5 days. It is catalase positive and indole positive. The most likely organism and interpretation is:
C. acnes is an aerotolerant anaerobe of skin that is catalase and indole positive and grows slowly. A single positive bottle usually means contamination. Clostridium and Actinomyces are catalase negative.
A technologist accidentally uses a 2.0 McFarland suspension instead of 0.5 McFarland for disk diffusion. What effect is expected?
A heavier inoculum reaches visible density before the drug has diffused far, so zones are smaller and results can appear falsely resistant. A too-light inoculum causes the opposite error.
Routine Mueller-Hinton disk diffusion plates for Enterobacterales are incubated in ambient air rather than 5% CO2 because CO2:
CO2 forms carbonic acid, lowering the pH; this reduces the activity of aminoglycosides and macrolides and increases that of some other drugs. Incubation in CO2 is used only for organisms that need it, with specific breakpoints.
A Streptococcus pneumoniae isolate from sputum shows a 23 mm zone around a 1 µg oxacillin disk. How should this be interpreted for penicillin?
An oxacillin zone of 20 mm or more predicts penicillin susceptibility in pneumococci. Zones below 20 mm need a penicillin MIC, because resistance is due to altered penicillin-binding proteins, not beta-lactamase.
A child with bloody diarrhoea develops hemolytic uremic syndrome. Which laboratory test detects all Shiga toxin-producing E. coli, including non-O157 serotypes?
Non-O157 STEC usually ferment sorbitol and are missed on SMAC. Detecting Shiga toxin or stx genes identifies all STEC serotypes.
An ESBL-producing Klebsiella pneumoniae typically remains susceptible in vitro to:
ESBLs hydrolyze third-generation cephalosporins and aztreonam but not cephamycins such as cefoxitin (and not carbapenems). Cefoxitin resistance suggests AmpC or porin loss.
A neutropenic patient with a central line has several positive blood cultures with a catalase-positive diphtheroid that grows better when lipid is added. It is resistant to most antibiotics except vancomycin. The most likely organism is:
C. jeikeium is lipophilic, multidrug resistant and causes line-related bacteraemia in immunocompromised patients; vancomycin is the drug of choice. Repeated isolation from several sets makes contamination unlikely.
According to CLSI, which disk is used in disk diffusion to predict mecA-mediated oxacillin resistance in Staphylococcus aureus?
Cefoxitin is a strong inducer of mecA and gives clearer, more reproducible results than oxacillin disks, so CLSI recommends the cefoxitin disk as the surrogate test for MRSA.
For group B streptococcus screening, CDC-recommended processing of a vaginal-rectal swab is:
Selective enrichment broth (e.g., Lim broth) increases detection of low numbers of GBS. Direct plating alone misses many carriers.
Blood cultures from a 68-year-old grow Streptococcus gallolyticus (formerly S. bovis biotype I). The laboratory result should prompt investigation for:
S. gallolyticus bacteremia or endocarditis is strongly associated with colonic tumours, so colonoscopy is recommended.
An aquarium keeper has a nodular skin lesion on the hand. Acid-fast bacilli are seen in a biopsy. For best recovery of the most likely organism, the laboratory should also incubate cultures at:
M. marinum causes 'fish tank granuloma' and grows best at 28–32 °C, often poorly at 37 °C. Skin and soft-tissue specimens should therefore also be cultured at about 30 °C.
The Xpert MTB/RIF assay detects rifampicin resistance by finding mutations in:
About 95% of rifampicin resistance is caused by mutations in the 81-bp rifampicin resistance-determining region of rpoB, which the assay probes. katG relates to isoniazid, gyrA to fluoroquinolones and pncA to pyrazinamide.
A mold culture at 25 °C shows large, thick-walled tuberculate macroconidia and small microconidia. This suggests:
Tuberculate (knobby) macroconidia are characteristic of the mold phase of Histoplasma capsulatum. Sepedonium species look similar, so conversion or molecular confirmation is needed.
Handling a mold culture of Coccidioides immitis on an open bench is dangerous mainly because it produces:
The mold phase forms alternating barrel-shaped arthroconidia that become airborne and cause laboratory infections. Suspected cultures must be handled in a biosafety cabinet under BSL-3 conditions.
A blue-green mold grows rapidly at 45 °C. Microscopy shows uniseriate phialides covering only the upper part of a club-shaped vesicle. The most likely species is:
A. fumigatus has uniseriate phialides on the upper two-thirds of the vesicle, bluish-green colonies and tolerates high temperature. A. niger has black colonies with biseriate phialides covering the whole vesicle.