MLT GlobeDOH exam preparation (Abu Dhabi) – page 24
700 practice MCQs for the DOH medical laboratory exam. Level: Basic to intermediate.
Activated protein C inactivates which coagulation factors?
APC, with protein S as cofactor, cleaves the activated cofactors Va and VIIIa. Factor V Leiden resists this cleavage.
Thawed FFP kept at 1–6 °C should be transfused within what period to retain its FFP labelling?
After 24 h the labile factor V and VIII levels fall; the unit may be relabelled thawed plasma and used within 5 days.
Serological typing of bacteria depends on surface or structural antigens. Which component is least useful for this purpose?
Capsular (K), cell-wall (O) and flagellar (H) antigens are exposed and strain-specific, forming the basis of serotyping; ribosomes are internal and highly conserved.
Which of these Gram-positive organisms is catalase-positive?
Aerobic spore-forming Bacillus produces catalase. Clostridium (anaerobic), Enterococcus and Streptococcus are catalase-negative.
Sputum from a transplant patient grows chalky colonies on BCYE agar in air after 6 days. Gram stain shows delicate branching, beaded Gram-positive rods that are partially acid-fast. The organism is most likely:
Aerobic growth, fine branching and modified acid-fast positivity point to Nocardia. Actinomyces is anaerobic and not acid-fast; Mycobacterium does not branch; Streptomyces is not acid-fast.
Both Nocardia and Actinomyces form branching Gram-positive rods. Which feature points to Actinomyces?
Actinomyces is anaerobic/microaerophilic and not acid-fast, producing sulphur granules in pus. Nocardia is aerobic and partially acid-fast.
Which non-culture method is recommended for diagnosing gonorrhoea in a woman using a vaginal or endocervical swab?
NAATs are the most sensitive and specific tests for N. gonorrhoeae in women. Cervical Gram stain is insensitive because of commensal Gram-negative flora; MALDI-TOF requires a cultured isolate.
A stool isolate is a non-lactose-fermenting Gram-negative rod that is motile and produces H2S on TSI. Which organism fits?
All Shigella species are non-motile and H2S-negative. Most Salmonella are motile and produce H2S.
A suprapubic urine is plated with a calibrated 0.01 mL (10 µL) loop and yields 10 colonies of Staphylococcus saprophyticus. What is the colony count?
CFU/mL = colonies x 1/loop volume = 10 x 100 = 1,000 CFU/mL. Any growth from suprapubic aspirate is significant.
Which infection is usually diagnosed by serology rather than culture or microscopy?
Coxiella is hazardous and hard to culture, so diagnosis relies on antibodies to phase I and II antigens. The other infections are diagnosed by culture, microscopy or NAAT.
Which genus is NOT an actinomycete (high-G+C Gram-positive group)?
Mycoplasma lacks a cell wall and belongs to the class Mollicutes. Mycobacterium, Nocardia and Actinomyces are actinomycetes.
A heat-sensitive endoscope from a TB clinic needs high-level disinfection. Which agent is suitable?
Glutaraldehyde is a high-level, tuberculocidal disinfectant for heat-sensitive instruments with adequate contact time. Quats and chlorhexidine are low-level and not tuberculocidal.
Bacterial flagella are too thin to see with routine stains. Which method makes them visible under the light microscope?
Flagella stains use tannic acid mordants to build up the flagellar thickness so it becomes visible. Albert's and Ponder's stains show volutin granules.
Which of these is an enriched special medium rather than a basic medium?
Bordet-Gengou contains potato, glycerol and blood for Bordetella pertussis. Peptone water, nutrient broth and meat extract agar are simple basal media.
Alpha-haemolytic colonies on blood agar become flattened with a sunken centre after 48 h, owing to autolysis. Which organism produces these 'draughtsman' colonies?
Pneumococcal autolysin causes central collapse, giving draughtsman or crater colonies. Enterococci and Moraxella form convex colonies.
Which organism is intrinsically resistant to vancomycin?
Vancomycin is too large to cross the Gram-negative outer membrane, so Enterobacterales are inherently resistant. Oral vancomycin is a treatment for C. difficile; MSSA and S. pyogenes are susceptible.
Neisseria meningitidis is least likely to cause which condition?
Meningococci cause meningitis, septicaemia and occasionally conjunctivitis, but they are not urinary tract pathogens.
Which bacterium shows characteristic tumbling motility at 25 °C but is poorly motile at 37 °C?
Listeria expresses flagella at room temperature, giving tumbling motility and an umbrella pattern in semi-solid agar. Y. pestis, Nocardia and Shigella are non-motile.
Neisseria gonorrhoeae is least commonly associated with which condition?
Gonococci commonly cause urethritis/cervicitis, disseminated infection with arthritis and ophthalmia neonatorum; endocarditis is rare.
Which statement about Clostridium tetani is INCORRECT?
Canned-food illness is botulism (C. botulinum). C. tetani is acquired through wounds and is an obligate anaerobe that grows in cooked meat medium.