MLT GlobeASCP exam preparation (USA · MLS / MLT) – page 42
1200 practice MCQs for the ASCP medical laboratory exam. Level: Advanced.
A technologist must pour and handle xylene, a volatile toxic solvent. Which engineering control is correct?
A chemical fume hood draws vapours away from the worker and exhausts them. Most Class II A2 cabinets recirculate air through HEPA filters, which do not trap chemical vapours.
After handling specimens from a patient with Clostridioides difficile infection, the best hand hygiene method is:
Alcohol does not kill bacterial spores; soap-and-water washing physically removes C. difficile spores. Alcohol rub is otherwise preferred for routine hand hygiene.
A blood alcohol (ethanol) level is ordered. Which skin preparation is correct?
Alcohol antiseptics could contaminate the sample and raise results, so a non-alcohol antiseptic is used. The site must still be cleaned to prevent infection.
A test is evaluated in 300 people: true positives 80, false negatives 20, true negatives 180, false positives 20. What is the negative predictive value?
NPV = TN / (TN + FN) = 180 / (180 + 20) = 0.90, or 90%. Sensitivity here is 80/(80+20) = 80%, which is a common confusion.
Under CLIA, for most quantitative chemistry tests without an approved alternative QC plan, the minimum control testing is:
The default CLIA requirement for quantitative tests is at least two levels of control material each day of patient testing. Weekly or monthly controls do not meet this minimum.
A laboratory lowers the cut-off value for a positive screening test. The expected effect is:
A lower cut-off labels more people positive, detecting more true cases (higher sensitivity) but also more false positives (lower specificity). The two usually move in opposite directions.
In a reverse CAMP test, an anaerobic gram-positive rod is streaked at right angles to Streptococcus agalactiae. An arrowhead of enhanced hemolysis points toward the rod. The rod is most likely:
The alpha toxin (lecithinase) of C. perfringens acts with the CAMP factor of group B streptococci to give an arrowhead of hemolysis pointing toward the Clostridium streak.
A patient without trauma develops rapidly spreading gas gangrene. Blood cultures grow a swarming, spore-forming anaerobic gram-positive rod that is lecithinase negative. The patient should be investigated for:
Clostridium septicum causes non-traumatic (spontaneous) gas gangrene and bacteremia and is strongly associated with colorectal cancer and neutropenia. It swarms and is lecithinase negative.
After 10 days of treatment for Clostridioides difficile infection, a patient's diarrhoea has stopped. The clinician asks for a repeat toxin test to prove cure. The best response is:
C. difficile spores, toxin and DNA can persist after clinical cure, so a test of cure is not recommended. Only unformed stool from symptomatic patients should be tested.
On egg yolk agar, an iridescent, pearly ('mother-of-pearl') layer on and around the colonies indicates:
Lipase breaks down free fats in egg yolk and forms an oily, iridescent sheen, as seen with C. botulinum and C. sporogenes. Lecithinase gives an opaque white zone in the medium.
The CLSI agar screen for vancomycin resistance in enterococci uses:
BHI agar containing 6 µg/mL vancomycin is spot-inoculated; growth of more than one colony suggests resistance. The salt-oxacillin agar screens staphylococci for oxacillin resistance.
Some blood culture bottles contain resins or activated charcoal. Their main purpose is to:
Resins and charcoal adsorb antibiotics, improving recovery from patients already on therapy. Growth detection is by separate CO2 sensors.
When a throat swab is plated on sheep blood agar for group A streptococci, the agar is stabbed several times with the loop. This is done to:
Streptolysin O is inactivated by oxygen, so haemolysis is best seen in the stab where oxygen is reduced. CAMP testing needs a S. aureus streak, not stabs.
A urethral swab for Neisseria gonorrhoeae culture will be delayed before plating. Which handling is correct?
N. gonorrhoeae is very sensitive to cold and drying; charcoal-containing transport medium at room temperature helps survival. Refrigeration kills the organism.
For disk diffusion testing, which organism must be prepared by the direct colony suspension method rather than the growth (broth culture) method?
CLSI requires direct colony suspension for fastidious organisms such as Haemophilus, streptococci and Neisseria, and for staphylococci when testing oxacillin. Non-fastidious gram-negative rods can use either method.
Disk diffusion testing of Streptococcus pneumoniae is performed on:
Pneumococci need blood supplementation and CO2 for adequate growth, and the incubation time is extended to 20–24 hours. Plain MH in air is used for Enterobacterales.
A laboratory reports carbapenem results for an E. coli only when the isolate is resistant to third-generation cephalosporins. This practice is called:
Cascade reporting shows broad-spectrum agents only when narrower agents are resistant, supporting antimicrobial stewardship. A cumulative antibiogram summarises many isolates over time.
A motile, lactose-fermenting gram-negative rod from blood is lysine decarboxylase positive, ornithine decarboxylase positive and arginine dihydrolase negative. The most likely organism is:
K. aerogenes (formerly Enterobacter aerogenes) is motile, LDC and ODC positive and ADH negative. E. cloacae is LDC negative and ADH positive; K. pneumoniae is non-motile and ODC negative.
A neonate fed powdered infant formula develops meningitis. The CSF grows a lactose-fermenting gram-negative rod that forms yellow-pigmented colonies at 25 °C. The most likely organism is:
Cronobacter sakazakii contaminates powdered infant formula and causes neonatal meningitis and sepsis. It produces a yellow pigment, best at 25 °C. Serratia produces red pigment.
In a Moeller decarboxylase test, both the control tube (no amino acid) and the lysine tube are purple after 24 hours under oil. The correct interpretation is:
The control tube must turn yellow to show glucose fermentation lowered the pH, which activates decarboxylases. A purple control means the test cannot be interpreted, often because the organism is a non-fermenter.