MLT GlobeASCP exam preparation (USA · MLS / MLT) – page 43
1200 practice MCQs for the ASCP medical laboratory exam. Level: Advanced.
A gram-negative rod is colourless on MacConkey agar and ONPG negative, but gives acid slant/acid butt with gas in TSI read at 20 hours. The best explanation is that the organism:
TSI contains sucrose as well as lactose. A lactose-negative, ONPG-negative organism that gives an acid slant after 18–24 hours must be fermenting sucrose, as some Proteus vulgaris and Yersinia strains do.
A patient has painless, beefy-red genital ulcers that bleed easily. A tissue smear shows Donovan bodies inside macrophages. The causative organism is:
Granuloma inguinale (donovanosis) is caused by Klebsiella granulomatis, seen as intracellular Donovan bodies with Giemsa or Wright stain. H. ducreyi causes painful soft ulcers (chancroid).
Which Klebsiella species is indole positive and linked to antibiotic-associated hemorrhagic colitis?
K. oxytoca is the indole-positive Klebsiella; toxin-producing strains cause hemorrhagic colitis after penicillin therapy. K. pneumoniae and K. aerogenes are indole negative.
Which ESBL enzyme family is now the most common worldwide in Escherichia coli?
CTX-M enzymes (especially CTX-M-15) are the dominant ESBLs worldwide. TEM-1 and SHV-1 are narrow-spectrum beta-lactamases, and KPC is a carbapenemase.
A patient develops septic shock during transfusion of a red cell unit stored for 30 days. A gram-negative rod that multiplies at 4 °C grows from the unit. The most likely organism is:
Y. enterocolitica is psychrotrophic and uses iron from stored red cells, so it multiplies during refrigerated storage and produces endotoxin. It is the classic cause of red cell transfusion sepsis.
A patient with advanced HIV has cavitary pneumonia. Sputum grows salmon-pink, mucoid colonies after 48 hours. The organisms are partially acid-fast gram-positive coccobacilli. The most likely organism is:
Rhodococcus equi forms salmon-pink mucoid colonies, is partially acid-fast and causes cavitary pneumonia in immunocompromised hosts, often after contact with horses or soil. Nocardia forms dry, chalky colonies with branching filaments.
After an eye injury with a soil-covered metal fragment, a patient develops rapidly progressive endophthalmitis. Vitreous fluid grows a large, beta-hemolytic, motile, spore-forming gram-positive rod that is lecithinase positive. The most likely organism is:
Bacillus cereus causes severe post-traumatic endophthalmitis. It is motile, beta-hemolytic and lecithinase positive. B. anthracis is non-motile and non-hemolytic; C. perfringens is anaerobic and non-motile.
A beta-hemolytic, catalase-negative coccus from a throat swab is susceptible to a 0.04 U bacitracin disk and resistant to trimethoprim-sulfamethoxazole (SXT). The presumptive identification is:
Group A streptococci are bacitracin susceptible and SXT resistant. Group B streptococci are resistant to both, while groups C and G are usually bacitracin resistant and SXT susceptible.
A broth microdilution test gives a vancomycin MIC of 4 µg/mL for a Staphylococcus aureus blood isolate. Using CLSI breakpoints, this result is interpreted as:
CLSI vancomycin breakpoints for S. aureus are ≤2 µg/mL susceptible, 4–8 µg/mL intermediate and ≥16 µg/mL resistant. An MIC of 4 µg/mL is therefore VISA, not VRSA.
The rapid pneumococcal urinary antigen test detects which component of Streptococcus pneumoniae?
The immunochromatographic urine test detects C-polysaccharide (cell wall teichoic acid) shared by all serotypes. It is not serotype specific and does not detect antibody.
An Enterococcus faecalis from urine appears susceptible to cefazolin in a disk diffusion test. How should this be reported?
Enterococci are intrinsically resistant to cephalosporins because of low-affinity penicillin-binding proteins. These drugs may look active in vitro but fail clinically, so CLSI says they must not be reported as susceptible.
An alpha-hemolytic streptococcus gives a 10 mm zone around a 6 mm optochin disk incubated in 5% CO2. What is the next step?
A zone ≥14 mm around a 6 mm disk presumptively identifies S. pneumoniae. Smaller zones are equivocal, because some pneumococci are optochin resistant, so bile solubility is used to confirm.
In the CAMP test, a Staphylococcus aureus streak is used because it produces:
The CAMP factor of group B streptococci acts together with staphylococcal beta-lysin to give an arrowhead of enhanced hemolysis. Coagulase and DNase play no part in this reaction.
A Ziehl-Neelsen sputum smear shows 35 acid-fast bacilli in 100 oil-immersion fields. Using the WHO/IUATLD grading scale, the result is reported as:
On the WHO/IUATLD scale, 10–99 AFB per 100 fields is 1+. Scanty is 1–9 AFB per 100 fields; 2+ needs 1–10 AFB per field.
Middlebrook 7H10 and 7H11 agar plates for mycobacterial culture should be incubated at 35–37 °C in:
Middlebrook agar media need 5–10% CO2 for best growth and are kept away from light. Anaerobic or microaerophilic atmospheres do not support M. tuberculosis.
A slow-growing mycobacterium from sputum grows well at 42 °C and is linked to hospital hot-water systems. The most likely species is:
M. xenopi is thermophilic, grows at 42–45 °C and lives in hot-water pipes. M. ulcerans, M. marinum and M. haemophilum prefer lower temperatures (about 30 °C).
A child from West Africa has a large painless skin ulcer with undermined edges. Acid-fast bacilli are seen. The likely agent is best cultured at:
Buruli ulcer is caused by M. ulcerans, which grows only at about 25–33 °C and very slowly (6–12 weeks). Culture at 37 °C often fails.
Most Mycobacterium tuberculosis strains give which result in the heat-stable (68 °C) catalase test?
M. tuberculosis catalase is heat-labile, so the 68 °C test is negative, whereas most NTM keep activity. Loss of catalase also occurs in some isoniazid-resistant (katG) strains.
Which urine specimen is recommended for mycobacterial culture?
Three consecutive first-morning specimens give the best yield. Pooled 24-hour urine is rejected because of contamination overgrowth and dilution.
Which microbiological finding meets the ATS/IDSA criterion for nontuberculous mycobacterial lung disease in a patient with compatible symptoms and imaging?
NTM are common in the environment, so one sputum isolate may be contamination. Two separate positive sputa (or one bronchial wash/lavage) with the same species are required.