MLT GlobeASCP exam preparation (USA · MLS / MLT) – page 44
1200 practice MCQs for the ASCP medical laboratory exam. Level: Advanced.
A child has scaly patches of hair loss with 'black dots' where hairs broke at the scalp. Wood's lamp shows no fluorescence, and spores fill the inside of the hair shaft. The most likely dermatophyte is:
T. tonsurans causes endothrix 'black dot' tinea capitis and does not fluoresce. Microsporum canis and M. audouinii are ectothrix and give green fluorescence.
A patient has persistent gonococcal urethritis after ceftriaxone treatment. The initial diagnosis was made by NAAT. What should the laboratory recommend?
NAATs do not provide an isolate for susceptibility testing. Suspected treatment failure needs culture so that antimicrobial resistance, including to ceftriaxone, can be tested and reported.
A nasopharyngeal isolate is an oxidase-positive gram-negative diplococcus that grows on modified Thayer-Martin agar. It produces acid from glucose and maltose and is ONPG positive. The most likely organism is:
N. lactamica resembles N. meningitidis but is beta-galactosidase (ONPG) positive and uses lactose. N. meningitidis is ONPG negative; M. catarrhalis is asaccharolytic.
A Haemophilus isolate on a medium without X or V factors grows around the V disk and the XV disk, but not around the X disk. The most likely species is:
Growth with V alone shows a V-only requirement, as in H. parainfluenzae. H. influenzae and H. haemolyticus need both X and V factors; H. ducreyi needs X only.
When Brucella is suspected in a blood culture, what should the clinician do?
Brucella is one of the most common causes of laboratory-acquired infection. Early warning lets staff use a biosafety cabinet and avoid aerosol-producing steps such as sniffing plates.
Which growth result separates Vibrio cholerae from most other pathogenic Vibrio species?
V. cholerae (and V. mimicus) can grow without added salt, whereas most other pathogenic vibrios, such as V. parahaemolyticus and V. vulnificus, are halophilic and need NaCl.
In Hugh-Leifson oxidation-fermentation (OF) glucose medium, the open tube turns yellow and the oil-sealed tube remains green. The organism is:
Acid only in the open, aerobic tube means glucose is used oxidatively, as by Pseudomonas. Fermenters make acid in both tubes; asaccharolytic organisms leave both tubes green or blue.
A patient treated for Helicobacter pylori needs a test to confirm eradication. The urea breath test or stool antigen test should be done at least:
Antibiotics and bismuth suppress H. pylori and can give false-negative results. Testing at least 4 weeks after therapy (and 2 weeks off proton pump inhibitors) avoids this.
Selective media for Campylobacter, such as charcoal cefoperazone deoxycholate agar (CCDA), suppress stool flora mainly by using:
Campylobacter media contain antibiotics, such as cefoperazone with other agents, to inhibit normal stool flora. Charcoal removes toxic oxygen products. High salt and alkaline pH are used for Vibrio media.
A traveller returns from a safari in East Africa with a painful skin chancre, high fever and enlarged lymph nodes. A thin blood film shows slender flagellates with an undulating membrane and a small subterminal kinetoplast. The organism is:
East African sleeping sickness (T. b. rhodesiense) produces many trypomastigotes in blood, with a small kinetoplast. T. cruzi has a large kinetoplast and a C-shaped form and occurs in the Americas.
Which laboratory method is most sensitive for confirming varicella-zoster virus in a vesicular skin lesion?
PCR of vesicle material is the most sensitive and specific test and distinguishes VZV from HSV. VZV is labile and grows poorly in culture, and the Tzanck smear cannot tell VZV from HSV.
The CSF/serum albumin ratio (albumin index) is used mainly to assess:
Albumin is made only in the liver, so a raised CSF/serum albumin ratio shows leakage across the blood-brain barrier. Intrathecal IgG production is assessed by the IgG index.
A cytocentrifuge slide of CSF shows macrophages containing dark blue-black haemosiderin granules. This most suggests:
Siderophages form when macrophages break down red cells over about 2 days or more, indicating earlier subarachnoid haemorrhage. A traumatic tap is too recent to produce them.
CSF is diluted 1:2 with diluting fluid. WBCs counted in the four large corner squares of a Neubauer chamber (each 1 mm² × 0.1 mm) total 36. The WBC count is:
Count × dilution ÷ volume counted = 36 × 2 ÷ (4 × 0.1 µL) = 180 cells/µL. Forgetting the dilution gives 90.
A patient with liver cirrhosis is confused and drowsy. Which CSF finding is most useful to support hepatic encephalopathy?
Ammonia detoxified in the brain forms glutamine, so CSF glutamine rises in hepatic encephalopathy. Low glucose and high lactate suggest bacterial meningitis.
Which diluent should NOT be used for a manual WBC count on synovial fluid?
Acetic acid precipitates hyaluronate-protein complexes (a mucin clot), trapping cells and giving false results. Saline, hypotonic saline or saponin can be used, the last two to lyse red cells.
A synovial fluid is so viscous that it cannot be pipetted accurately for a cell count. What is the recommended pretreatment?
Hyaluronidase breaks down hyaluronic acid and lowers viscosity without damaging cells. Acetic acid forms a mucin clot, and counting supernatant would miss the cells.
In synovial fluid from rheumatoid arthritis, neutrophils with dark cytoplasmic granules containing immune complexes are called:
Ragocytes contain ingested immune complexes (including rheumatoid factor) that look like dark granules. Reiter cells are macrophages that have ingested neutrophils.
A sexually active 22-year-old has migratory joint pain, tenosynovitis and skin pustules. A knee aspirate is sent for culture. Which organism is suspected, and on which medium should it be plated promptly?
This is the arthritis–dermatitis syndrome of disseminated gonococcal infection. N. gonorrhoeae is fastidious, so the fluid should be plated quickly on chocolate agar, avoiding refrigeration.
Pleural fluid LD is 300 U/L and serum LD is 250 U/L. Using Light's criteria, how is the fluid classified?
300/250 = 1.2, which is greater than 0.6, so the fluid meets an LD criterion for an exudate. Meeting any one of Light's criteria is enough.