MLT GlobeASCP exam preparation (USA · MLS / MLT) – page 30
1200 practice MCQs for the ASCP medical laboratory exam. Level: Advanced.
On Hektoen enteric agar, a stool isolate forms blue-green colonies with black centres. This appearance is most typical of:
Salmonella does not ferment the sugars in HE agar (blue-green colony) and produces H2S (black centre). Shigella is blue-green without black centres, and E. coli ferments lactose giving orange-salmon colonies.
An E. coli from blood has a ceftriaxone MIC of 2 µg/mL. The CLSI breakpoints are: susceptible ≤1, intermediate 2, resistant ≥4 µg/mL. How should the result be reported?
An MIC of 2 µg/mL falls exactly on the intermediate category for ceftriaxone in Enterobacterales. Ceftriaxone has no susceptible-dose dependent category in these breakpoints.
In CLSI reports, the category 'susceptible-dose dependent' (SDD) means that:
SDD means the isolate can be treated successfully if a dosing regimen giving higher drug exposure is used, such as higher-dose cefepime for Enterobacterales. It is not a technical error flag.
In an MBC test, broth from clear MIC wells is subcultured. The minimum bactericidal concentration is the lowest concentration that:
MBC is the lowest concentration that produces a 3-log (99.9%) or greater reduction in viable count on subculture. No visible turbidity only defines the MIC.
Which Staphylococcus aureus QC strain is recommended by CLSI for broth microdilution MIC testing?
S. aureus ATCC 29213 is the MIC QC strain, while ATCC 25923 is used for disk diffusion. ATCC 43300 is an MRSA control strain, and ATCC 700603 is an ESBL-producing K. pneumoniae.
A motile lactose fermenter from a wound has IMViC – – + + and is ornithine decarboxylase positive. It is most likely:
Enterobacter cloacae is VP and citrate positive, motile and ornithine positive. K. pneumoniae shares the IMViC pattern but is non-motile and ornithine negative.
A teenager with right lower quadrant pain has a stool isolate forming 'bull's-eye' colonies on CIN agar. It is motile at 25 °C but not at 37 °C. The organism is:
Y. enterocolitica gives red-centred colonies on cefsulodin-irgasan-novobiocin agar and is motile only at 22–25 °C. It can mimic appendicitis (mesenteric lymphadenitis).
A bubo aspirate from a patient with fever after rodent exposure shows gram-negative bacilli with bipolar 'safety-pin' staining. The laboratory should suspect:
Y. pestis shows bipolar staining with Wayson or Giemsa stains. It is a select agent, so work is done in a biosafety cabinet and referred to a reference laboratory.
A green colour after adding 10% ferric chloride to a phenylalanine agar slant indicates deamination. A positive result is characteristic of:
Phenylalanine deaminase converts phenylalanine to phenylpyruvic acid, which gives a green colour with ferric chloride. This is a key trait of the Proteeae tribe.
A TSI slant shows alkaline slant/alkaline butt (K/K) with no H2S after 24 hours. This suggests the isolate:
All Enterobacterales ferment glucose and give at least an acid butt. A K/K reaction indicates a non-fermenter such as Pseudomonas or Alcaligenes.
A newborn's CSF grows small beta-haemolytic colonies that look like group B streptococci. The Gram stain shows short gram-positive rods and coccobacilli. Which quick test best separates Listeria from Streptococcus agalactiae?
Listeria is catalase positive, while streptococci are catalase negative. Both organisms can be CAMP positive, hippurate positive and beta-haemolytic, so those tests do not separate them.
Which test best separates Enterococcus spp. from Streptococcus gallolyticus (group D non-enterococcus)?
Both groups hydrolyze bile esculin, but only enterococci grow in 6.5% NaCl. Both are catalase negative and optochin resistant.
A catalase-positive gram-positive coccus in tetrads forms bright yellow colonies, is modified oxidase positive and susceptible to 0.04 U bacitracin. It is most likely:
Micrococcus is modified oxidase (microdase) positive and bacitracin susceptible; staphylococci are negative and resistant. Aerococcus is catalase negative.
A large-colony beta-hemolytic streptococcus must be separated from S. dysgalactiae (group C/G). Which single test is most specific for S. pyogenes among these?
Among beta-hemolytic streptococci, PYR positivity is specific for S. pyogenes. Some group C and G strains are bacitracin susceptible, so bacitracin is less specific.
Rapid latex agglutination kits for identifying Staphylococcus aureus usually detect:
Latex particles coated with fibrinogen and IgG bind clumping factor and protein A on the S. aureus surface. Free coagulase is detected by the tube test.
In liquid culture, Mycobacterium tuberculosis grows as parallel, rope-like 'serpentine cords'. This appearance is linked to:
Cord factor (trehalose 6,6'-dimycolate) makes the bacilli stick side by side in serpentine cords, a presumptive sign of the M. tuberculosis complex. Niacin accumulation is a biochemical test, not a morphology.
A slow-growing acid-fast bacillus forms cream colonies in the dark, which turn yellow after exposure to light. Which species fits this Runyon group?
Pigment formed only after light exposure defines photochromogens (Runyon group I), such as M. kansasii. M. gordonae is a scotochromogen (pigmented in the dark), M. avium is a non-chromogen and M. fortuitum is a rapid grower.
A patient with advanced HIV (CD4 count below 50 cells/µL) has fever, weight loss and anaemia. Which specimen is most useful to diagnose the most likely disseminated non-tuberculous mycobacterial infection?
Disseminated Mycobacterium avium complex is common at very low CD4 counts and causes continuous bacteraemia, so blood cultured in mycobacterial media is the best specimen. Routine urine culture media do not support mycobacteria.
A new TB patient's sputum gives 'MTB detected; rifampicin resistance detected' on Xpert MTB/RIF. The most appropriate interpretation is:
The result shows rifampicin-resistant TB, which is managed as MDR/RR-TB. Further tests, such as culture-based or molecular testing for isoniazid and fluoroquinolones, are needed to guide treatment. Xpert does not test isoniazid.
Which medium produces brown to black colonies of Cryptococcus neoformans because of phenoloxidase activity?
Niger seed (birdseed) agar contains caffeic acid, which C. neoformans phenoloxidase converts to melanin, giving brown colonies. Cornmeal agar is for morphology, and Mycosel contains cycloheximide, which inhibits Cryptococcus.