MLT GlobeSCFHS exam preparation (Saudi Arabia) – page 25
749 practice MCQs for the SCFHS medical laboratory exam. Level: Intermediate.
The ONPG test is useful to identify 'late' lactose fermenters because these organisms:
Late fermenters have beta-galactosidase but lack permease, so lactose enters slowly. ONPG enters the cell without permease and is split to yellow o-nitrophenol.
A known Shigella control tested in TSI shows an acid slant/acid butt when read after 8 hours. The most likely explanation is:
Early in incubation glucose fermentation acidifies the whole tube. The slant becomes alkaline only after glucose is used and peptones are oxidized, so TSI must be read at 18–24 hours.
A K. pneumoniae from a community-acquired liver abscess forms a viscous string longer than 5 mm when a colony is lifted with a loop. This suggests:
A positive string test marks the hypermucoviscous phenotype linked to hypervirulent K. pneumoniae causing liver abscess and metastatic infection. It does not indicate resistance.
A throat isolate forms black colonies surrounded by a brown halo on Tinsdale agar. This appearance suggests:
Tinsdale agar contains potassium tellurite and cystine; C. diphtheriae reduces tellurite (black colony) and has cystinase (brown halo). Most other corynebacteria lack the brown halo.
A large gram-positive rod from a wool worker's skin lesion forms flat, ground-glass colonies with 'Medusa head' edges. Which pair of results best suggests Bacillus anthracis rather than B. cereus?
B. anthracis is non-haemolytic on sheep blood agar and non-motile, whereas B. cereus is beta-haemolytic and motile. A suspected isolate should be referred to a reference laboratory.
The tube coagulase test for Staphylococcus aureus detects which product?
The tube test detects free (extracellular) coagulase, which activates prothrombin to form a clot in plasma. Bound coagulase (clumping factor) is detected by the slide test.
An S. aureus isolate is erythromycin resistant and clindamycin susceptible. In a D-zone test, the clindamycin zone is flattened on the side facing the erythromycin disk. What should be reported?
A D-shaped zone shows inducible MLSB resistance (erm gene). Clindamycin may fail during therapy, so it is reported as resistant.
A 9-year-old develops glomerulonephritis 3 weeks after impetigo. Which antibody test best supports recent group A streptococcal skin infection?
After skin infection the ASO response is often weak because skin lipids inactivate streptolysin O. Anti-DNase B rises reliably after both pharyngeal and skin infections.
A newborn has widespread blistering and peeling of skin with a positive Nikolsky sign; blister fluid is sterile. The toxin involved acts on:
Staphylococcal exfoliative toxins cleave desmoglein-1, causing scalded skin syndrome. The toxin is produced at a distant site, so blister fluid is usually sterile.
In NALC-NaOH processing of sputum for mycobacterial culture, the role of N-acetyl-L-cysteine (NALC) is to:
NALC is a mucolytic that breaks disulfide bonds in mucus. Sodium hydroxide is the decontaminating agent, and phosphate buffer is used to dilute and neutralize afterwards.
A yellow scotochromogenic, slow-growing mycobacterium is isolated once from a bronchial wash. It is known to be common in tap water and rarely causes disease. It is most likely:
M. gordonae, the 'tap water bacillus', is a scotochromogen and a frequent contaminant from water used in procedures or processing. A single isolate is usually not significant.
A positive liquid mycobacterial culture is tested with an immunochromatographic test for MPT64 antigen and gives a positive result. This indicates:
MPT64 is a protein secreted by M. tuberculosis complex organisms but not by most non-tuberculous mycobacteria, so the test rapidly identifies MTBC from positive cultures. Some BCG strains can be negative.
A corneal scraping from a contact-lens wearer grows a lavender-pink mold with sickle- or canoe-shaped multicelled macroconidia. The genus is:
Fusarium produces banana-shaped multiseptate macroconidia and often pink-purple colonies; it is a major cause of fungal keratitis. Acremonium has only single-celled conidia in slimy heads.
A dematiaceous mold shows brown, club-shaped conidia with both transverse and longitudinal septa, formed in chains. The genus is:
Alternaria forms drumstick-shaped muriform conidia in chains. Curvularia conidia are curved with transverse septa only, and Cladosporium produces small shield-shaped conidia.
Which Candida species is intrinsically resistant to fluconazole and should be reported with this comment?
Pichia kudriavzevii (C. krusei) is intrinsically resistant to fluconazole. The other species are usually susceptible, although acquired resistance can occur.
Serum galactomannan testing is mainly used to support the diagnosis of:
Galactomannan is a cell wall antigen of Aspergillus, used in serum or bronchoalveolar lavage from high-risk haematology patients. Mucorales do not release galactomannan.
Which laboratory test is most useful for rapid diagnosis of disseminated histoplasmosis in an immunocompromised patient?
Histoplasma polysaccharide antigen in urine (and serum) is highly sensitive in disseminated disease. Galactomannan can cross-react but is not the test of choice.
A gram-negative coccobacillus from a ventilated ICU patient's tracheal aspirate is oxidase negative, catalase positive and non-motile. It grows on MacConkey agar as a non-lactose fermenter. The most likely organism is:
Acinetobacter is an oxidase-negative, non-motile coccobacillus that grows on MacConkey and causes ventilator-associated pneumonia. S. maltophilia is also usually oxidase negative but is motile and a rod.
A returning traveller has fever, but the first blood films are negative for malaria. The best next step is:
One negative set does not rule out malaria; films should be repeated (with rapid antigen testing) until three sets are negative. Serology does not diagnose acute malaria.
A splenectomised patient from the north-eastern USA has intraerythrocytic ring forms with no pigment, extracellular forms, and occasional tetrads ('Maltese cross'). The likely organism is:
Babesia rings lack haemozoin pigment and schizonts, may be extracellular, and tetrads are diagnostic. Plasmodium produces pigment and schizonts.