MLT GlobeASCP exam preparation (USA · MLS / MLT) – page 54
1200 practice MCQs for the ASCP medical laboratory exam. Level: Advanced.
An M. tuberculosis complex isolate is niacin negative, nitrate negative, susceptible to thiophene-2-carboxylic acid hydrazide (TCH) and naturally resistant to pyrazinamide. It is most likely:
M. bovis (including BCG) is intrinsically pyrazinamide resistant, niacin and nitrate negative, and inhibited by TCH. M. tuberculosis is niacin and nitrate positive and TCH resistant.
A line probe assay detects a mutation in the inhA promoter of an M. tuberculosis isolate, with no katG mutation. This usually indicates low-level isoniazid resistance and cross-resistance to:
inhA encodes the target shared by isoniazid and ethionamide, so inhA promoter mutations give low-level isoniazid resistance and ethionamide cross-resistance. katG mutations give high-level isoniazid resistance without ethionamide cross-resistance.
The in vitro hair perforation test is used to separate:
T. mentagrophytes complex forms wedge-shaped perforations in hair in vitro, whereas T. rubrum does not. It is a classic test for these two look-alike dermatophytes.
A yeast from urine forms small cells only, with no pseudohyphae on cornmeal agar, and rapidly hydrolyses trehalose. The most likely identification is:
C. glabrata forms small blastoconidia without pseudohyphae and gives a rapid trehalose test. C. albicans and C. tropicalis form pseudohyphae; C. krusei forms elongated 'crossed-matchstick' pseudohyphae.
A yeast-like isolate from blood produces arthroconidia and blastoconidia on cornmeal agar and is urease positive. The most likely genus is:
Trichosporon produces both arthroconidia and blastoconidia and is urease positive. Geotrichum forms arthroconidia without blastoconidia and is urease negative.
Serum (1,3)-beta-D-glucan is positive in many invasive fungal infections but is characteristically LOW or negative in infection with:
Mucorales contain little beta-D-glucan and Cryptococcus is masked by its capsule, so the test is often negative. Pneumocystis, Candida and Aspergillus usually give positive results.
A rice farmer from Thailand has pneumonia and liver abscesses. Blood cultures grow an oxidase-positive, motile gram-negative rod with bipolar staining and dry, wrinkled colonies at 48 hours. It is resistant to gentamicin and colistin but susceptible to amoxicillin-clavulanate. The most likely organism is:
This is melioidosis. B. pseudomallei is typically gentamicin and colistin resistant but amoxicillin-clavulanate susceptible, the reverse of most P. aeruginosa. B. mallei is non-motile. Work should be done in a biosafety cabinet.
Carbapenem resistance in Acinetobacter baumannii worldwide is most often caused by:
Acquired class D OXA carbapenemases (OXA-23, OXA-24/40, OXA-58) are the main cause of carbapenem resistance in A. baumannii. Loss of OprD porin is the classic mechanism in P. aeruginosa, not Acinetobacter.
In a patient with Plasmodium falciparum infection, finding mature schizonts in the peripheral blood is significant because it usually indicates:
Mature P. falciparum stages normally sequester in deep capillaries, so peripheral schizonts suggest heavy infection and a worse prognosis.
A malaria rapid test based on HRP2 is negative, but films show P. falciparum. A known cause of this false-negative result is:
Some P. falciparum strains lack the hrp2/hrp3 genes and produce no HRP2 antigen, so HRP2-based RDTs miss them. Very high parasitemia (prozone) can also cause false negatives.
A pregnant woman has positive Toxoplasma IgG and IgM. Which test best helps show the infection occurred more than 4 months ago?
High IgG avidity indicates infection at least 3–5 months earlier, because IgM can persist for months or years. Low avidity supports recent infection.
A trichrome-stained stool shows small trophozoites with one or two fragmented nuclei (chromatin in 4–6 granules) and no cyst stage is seen. The organism is:
Dientamoeba fragilis is a flagellate without external flagella, often binucleate with fragmented chromatin, and seen only in permanent stains. Endolimax nana has a single blot-like karyosome.
A large yellow-brown egg (about 140 × 80 µm) with an indistinct operculum is found in a patient who eats watercress. The parasite is most likely:
Fasciola eggs are among the largest helminth eggs, with a small indistinct operculum; infection follows eating contaminated aquatic plants. Fasciolopsis eggs look the same, so history is important.
A chronic HBV patient is HBeAg negative and anti-HBe positive, but HBV DNA is high and ALT is raised. The most likely explanation is:
Precore or core promoter mutations reduce HBeAg production while the virus keeps replicating. HBV DNA, not HBeAg, reflects replication in these patients.
Why is it important to tell HIV-2 from HIV-1 infection?
HIV-2 is naturally resistant to non-nucleoside reverse transcriptase inhibitors, and many HIV-1 RNA assays do not quantify it. Treatment and monitoring must therefore differ.
A premature neonate has yellow (xanthochromic) CSF with a normal cell count and no signs of hemorrhage. The most likely explanation is:
Neonates, especially premature ones, have a more permeable blood–brain barrier and often high bilirubin, giving xanthochromia without bleeding. A traumatic tap gives red, not yellow, fluid initially.
Undiluted CSF is counted in a Neubauer hemocytometer. A total of 45 WBCs is counted in all 9 large squares (each 1 mm² × 0.1 mm deep). The WBC count is:
Volume counted = 9 × 0.1 µL = 0.9 µL. Cells/µL = 45 ÷ 0.9 × dilution factor 1 = 50 cells/µL.
A traumatic tap gives CSF RBC 7000/µL and WBC 30/µL. Blood counts are normal. Using the correction of 1 WBC per 700 RBC, the corrected CSF WBC is:
Blood-derived WBCs = 7000 ÷ 700 = 10/µL; corrected WBC = 30 − 10 = 20/µL. This simple correction applies only when the peripheral blood count is normal.
Which anticoagulant is preferred for a synovial fluid tube used for crystal examination?
Sodium heparin is preferred; lithium heparin, powdered EDTA and oxalate can form crystals that may be mistaken for pathological ones.
Pleural fluid pH is requested for a parapneumonic effusion. Which handling is correct?
Air exposure lets CO2 escape and raises pH, so the sample must be anaerobic and measured with a blood gas analyser. A pH below 7.2 suggests a complicated effusion needing drainage.