MLT GlobeASCP exam preparation (USA · MLS / MLT) – page 59
1200 practice MCQs for the ASCP medical laboratory exam. Level: Advanced.
In adults, antibodies against bacterial capsular polysaccharides, such as pneumococcal capsule, are mainly of which IgG subclass?
Polysaccharide antigens mostly induce IgG2, so IgG2 deficiency predisposes to encapsulated bacterial infections. Protein antigens mainly induce IgG1 and IgG3.
A child has haemolytic anaemia, thrombocytopenia and acute kidney injury without preceding diarrhoea. C3 is low and C4 is normal. Which defect is most likely?
Atypical haemolytic uraemic syndrome is often due to factor H defects, causing uncontrolled alternative pathway activation on endothelium (low C3, normal C4). C1 inhibitor deficiency causes angioedema with low C4.
A patient with glomerulonephritis has persistently very low C3 with normal C4. An autoantibody that stabilises the alternative pathway C3 convertase is found. This autoantibody is:
C3 nephritic factor binds C3bBb and protects it from decay, causing continuous C3 consumption; it is linked to C3 glomerulopathy. Anti-C1q antibodies relate to lupus nephritis and lower C4 as well.
A patient's HIV-1/2 antigen/antibody combination assay is reactive, but the HIV-1/HIV-2 antibody differentiation immunoassay is negative. The next step is:
A reactive screen with negative differentiation may be acute HIV-1 infection (antigen present before antibody) or a false-positive screen. An HIV-1 RNA test resolves this; reporting negative could miss acute infection.
In the differential absorption (Davidsohn) test, the heterophile antibody of infectious mononucleosis is:
The infectious mononucleosis heterophile antibody is not a Forssman antibody, so it is not removed by guinea pig kidney but is removed by beef red cells. Forssman antibodies are removed by guinea pig kidney.
A patient taking high-dose biotin has testing on streptavidin-biotin immunoassays. Which effect is expected?
Excess free biotin blocks streptavidin binding of the biotinylated complex. In sandwich assays signal falls (falsely low); in competitive assays low signal is read as high analyte (falsely high).
After allogeneic stem cell transplantation, donor engraftment is best monitored in the laboratory by:
STR analysis compares donor- and recipient-specific DNA markers in blood or marrow to measure the percentage of donor cells. ABO typing may not change if donor and recipient share the same group.
A 5-year-old with an unsteady gait and small dilated vessels on the conjunctivae has recurrent sinopulmonary infections and IgA deficiency. Which serum test is typically raised?
Ataxia-telangiectasia (ATM gene) shows ataxia, telangiectasia, IgA/IgG2 deficiency and a characteristically raised serum AFP. IgE is usually low or normal in this disorder.
A chloridometer generates silver ions at a constant rate; the time to reach the endpoint is proportional to the chloride concentration. This method is:
In coulometric-amperometric titration, generated Ag⁺ combines with Cl⁻; excess Ag⁺ at the endpoint increases current and stops the timer. ISE methods measure a voltage instead.
A culture suspected to contain Brucella is shipped to a reference laboratory as a Category A infectious substance. Which UN number applies?
Category A substances infecting humans are shipped as UN 2814. Category B biological substances use UN 3373, UN 3291 is clinical waste and UN 1845 is dry ice.
A laboratory continuously calculates the average of patient results for sodium to detect analytical shifts between control runs. This approach is called:
Moving averages of patient results can reveal systematic shifts between control runs. A delta check compares one patient's current result with that patient's previous result.
Daptomycin MICs are falsely high if the broth is not supplemented with:
Daptomycin needs free calcium to bind to the bacterial membrane, so CLSI requires Mueller-Hinton broth adjusted to 50 mg/L calcium. Lysed horse blood is used for streptococci.
Oxacillin and cefoxitin tests for Staphylococcus aureus should be incubated at 33–35 °C and not above 35 °C because:
Expression of mecA-mediated resistance is temperature dependent, and heteroresistant subpopulations may be missed above 35 °C. S. aureus grows well at 37 °C, so poor growth is not the reason.
When reading disk diffusion zones for Proteus mirabilis, a thin veil of swarming growth extends into the zone of inhibition. According to CLSI, the technologist should:
CLSI states that a thin veil of swarming Proteus should be ignored and the zone read at the clear margin of heavy growth. MacConkey is not a standard AST medium.
A non-swarming, lactose-negative urine isolate is phenylalanine deaminase positive, strongly urease positive, H2S negative, ornithine decarboxylase positive and citrate negative. The most likely organism is:
Morganella morganii is PAD and urease positive, ODC positive, H2S negative and does not swarm. Proteus species swarm and produce H2S; Providencia stuartii is ODC negative and citrate positive.
A traveller who ate raw oysters has diarrhoea. The stool grows an oxidase-positive, glucose-fermenting gram-negative rod that is lysine and ornithine decarboxylase positive, arginine dihydrolase positive and DNase negative. The most likely organism is:
Plesiomonas shigelloides is the only oxidase-positive member of the Enterobacterales and is positive for all three decarboxylase/dihydrolase tests. Aeromonas is DNase positive; Shigella is oxidase negative.
A wound infection follows an injury while cleaning freshwater fish. The isolate is lactose negative, H2S positive, indole positive, lysine decarboxylase positive and urease negative. The most likely organism is:
Edwardsiella tarda, found in fish and freshwater, is H2S and indole positive and LDC positive. Salmonella is indole negative, P. vulgaris is urease positive and LDC negative, and C. freundii is LDC negative.
A Proteus mirabilis isolate has an imipenem MIC of 2 µg/mL but is susceptible to meropenem and ertapenem, and the mCIM is negative. The most likely explanation is:
Proteus, Providencia and Morganella often have raised imipenem MICs by non-carbapenemase mechanisms (low PBP affinity). Normal meropenem and ertapenem results and a negative mCIM argue against a carbapenemase.
Which result helps separate Corynebacterium diphtheriae from most other Corynebacterium species?
C. diphtheriae is pyrazinamidase negative (and cystinase positive), while most other corynebacteria are pyrazinamidase positive. Catalase, lack of motility and club shape are shared by most corynebacteria.
Blood cultures from a patient with endocarditis show gram-positive cocci in chains. On subculture, colonies grow on sheep blood agar only as tiny satellites next to a Staphylococcus aureus streak. The most likely organism is:
Nutritionally variant streptococci (Abiotrophia, Granulicatella) need pyridoxal or L-cysteine, which S. aureus supplies, so they satellite. S. mitis and enterococci grow well on sheep blood agar alone.