MLT GlobeDOH exam preparation (Abu Dhabi) – page 25
700 practice MCQs for the DOH medical laboratory exam. Level: Basic to intermediate.
Which statement about Clostridium perfringens (C. welchii) is INCORRECT?
C. perfringens is non-motile, unlike most clostridia. It is capsulated in tissue, produces lecithinase (Nagler reaction) and seldom sporulates in vivo or in routine media.
A laboratory sets up an Elek immunodiffusion plate using a filter-paper strip soaked in antitoxin. What is being demonstrated?
The Elek test detects diphtheria toxin: toxigenic C. diphtheriae forms precipitin lines where toxin meets antitoxin. Salmonella Typhi antibodies are detected by the Widal test, not the Elek test.
Throat and skin 'diphtheroids' such as Corynebacterium xerosis and C. pseudodiphtheriticum (formerly C. hoffmannii) are distinguished from pathogenic C. diphtheriae mainly because they:
Commensal corynebacteria are gram-positive, non-acid-fast, non-sporing rods like C. diphtheriae; the key difference is lack of toxin production (Elek-negative). 'C. acnes' is now Cutibacterium acnes.
Biological indicators used to check steam autoclave performance contain spores of:
G. stearothermophilus spores are highly resistant to moist heat. B. atrophaeus spores are used for dry heat and ethylene oxide.
The Koch–Weeks bacillus, a cause of epidemic purulent conjunctivitis, is now classified as:
Koch–Weeks bacillus (formerly H. aegyptius) is now considered a biogroup of H. influenzae. H. ducreyi causes chancroid; K. granulomatis causes donovanosis.
Mycoplasmas lack a cell wall; their plasma membrane is stabilised by incorporating:
Mycoplasmas take up cholesterol from the host or medium into their membrane, which is why serum is added to culture media. They have no peptidoglycan or teichoic acid.
Which organism is NOT a sexually transmitted pathogen?
H. influenzae biogroup aegyptius causes conjunctivitis. H. ducreyi (chancroid), K. granulomatis (donovanosis) and T. pallidum (syphilis) are sexually transmitted.
Which Brucella species is associated mainly with goats and sheep and causes the most severe human brucellosis?
B. melitensis (goats, sheep) is the most virulent for humans. B. abortus is from cattle, B. suis from pigs, B. canis from dogs.
Clostridium tetani is best described as:
C. tetani is a motile, peritrichous anaerobe with round terminal spores ('drumstick'). C. perfringens is the non-motile clostridium.
Which toxin acts as a superantigen, linking MHC class II to T-cell receptors non-specifically?
TSST-1 of S. aureus is a superantigen causing massive cytokine release. Exotoxin A and diphtheria toxin inhibit EF-2; tetanospasmin blocks inhibitory neurotransmitter release.
A patient with Staphylococcus aureus bacteraemia is suspected of endocarditis or deep metastatic abscess. The key laboratory investigation is:
Persistent positive blood cultures indicate an endovascular or metastatic focus. ASO is for streptococci and Widal for typhoid; S. aureus has no endotoxin.
Drops of serial dilutions are placed on an agar surface and colonies in the drop areas counted to estimate CFU/mL. This is the:
The Miles and Misra surface-drop technique estimates viable counts. ELISA and Widal detect antigen or antibody.
Most clinical isolates of which organism are resistant to benzylpenicillin because they produce penicillinase?
Over 80–90% of S. aureus strains produce beta-lactamase. S. pyogenes and T. pallidum remain penicillin-susceptible.
Which test best separates Micrococcus from coagulase-negative staphylococci?
Micrococcus contains cytochrome c and is microdase-positive; staphylococci are negative. Both genera are catalase-positive gram-positive cocci.
Gram-positive cocci that are larger than staphylococci and typically arranged in tetrads are:
Micrococci are about 1–1.8 µm and often form tetrads, and are commonly skin contaminants. Staphylococci form clusters; streptococci chains.
Which statement about bacterial plasmids is INCORRECT?
Many toxins are plasmid-encoded, e.g. anthrax toxin and some E. coli enterotoxins. Plasmids also carry resistance genes.
The alpha toxin of Clostridium perfringens damages cell membranes because it is a:
Lecithinase hydrolyses membrane phospholipids, causing haemolysis and tissue necrosis; it gives opalescence on egg-yolk agar (Nagler reaction).
Which feature is NOT typical of mycetoma (Madura foot)?
Mycetoma is a chronic painless swelling with sinuses discharging grains, caused by fungi or actinomycetes. Systemic fever is not typical.
Phenol is bactericidal (though not sporicidal) at a concentration of about:
Phenol at around 1% kills vegetative bacteria and is the reference in the phenol coefficient test. Lower concentrations are merely bacteriostatic.
Which organism is NOT in the order Enterobacterales?
Campylobacter is a curved, oxidase-positive, microaerophilic rod. Klebsiella, Providencia and Serratia are Enterobacterales.