MLT GlobeQCHP exam preparation (Qatar) – page 7
749 practice MCQs for the QCHP medical laboratory exam. Level: Intermediate.
Which syphilis test is a treponemal test?
TP-PA detects antibodies to Treponema pallidum antigens. RPR, VDRL and USR detect non-specific antibodies to cardiolipin-lecithin-cholesterol antigen (non-treponemal tests).
Which condition is a common cause of a biological false-positive RPR?
Anti-phospholipid (cardiolipin) antibodies in SLE, pregnancy, some infections and older age can react with the RPR antigen. A treponemal test will be non-reactive.
A 6-month-old boy has recurrent bacterial infections. Flow cytometry shows almost no CD19+ B cells, normal T cells and very low all immunoglobulin classes. The most likely diagnosis is:
Mutation in BTK blocks B-cell maturation, so B cells and all immunoglobulins are absent. Infections start after maternal IgG wanes around 6 months.
An infant with neonatal hypocalcaemia, a heart defect and low T-cell numbers most likely has:
22q11.2 deletion causes abnormal third and fourth pharyngeal pouch development: thymic hypoplasia (low T cells), absent parathyroids (hypocalcaemia) and conotruncal heart defects.
A CSF sample for bacterial culture from a patient with suspected meningitis should be:
Organisms such as Neisseria meningitidis and Haemophilus influenzae are killed by cold. CSF for bacterial culture must never be refrigerated and should be processed immediately as a stat specimen.
The minimum inhibitory concentration (MIC) is defined as the:
The MIC is the lowest concentration in a dilution series that inhibits visible growth after standard incubation. Killing 99.9% of the inoculum defines the minimum bactericidal concentration (MBC).
Which set of characteristics is typical of members of the order Enterobacterales?
Enterobacterales ferment glucose, reduce nitrate to nitrite and are oxidase negative (Plesiomonas is the main oxidase-positive exception). Oxidizers such as Pseudomonas are non-fermenters.
A lactose non-fermenter from a respiratory specimen produces a red pigment at room temperature, is DNase positive and hydrolyzes gelatin. It is most likely:
S. marcescens produces prodigiosin (red pigment, best at 22–25 °C) and is DNase and gelatinase positive. Many strains are not pigmented at 37 °C.
'Cold enrichment' at 4 °C to recover an organism from mixed specimens such as food or placenta is used for:
Listeria grows at refrigerator temperature while most competing flora do not, so holding the sample at 4 °C enriches it. This also explains its spread through refrigerated foods.
Swelling of the capsule of an organism when mixed with type-specific antiserum and methylene blue is called the:
The Quellung (Neufeld) reaction is capsular swelling with specific antiserum and is classically used to serotype S. pneumoniae. CAMP relates to group B streptococci.
Menstrual toxic shock syndrome linked to tampon use is mainly caused by:
Toxic shock syndrome toxin-1 is a superantigen of S. aureus and causes most menstrual TSS. Exfoliative toxins cause scalded skin syndrome.
The main advantage of auramine-rhodamine fluorochrome staining over Ziehl-Neelsen staining for acid-fast bacilli is:
Fluorescent bacilli are easy to see at lower power, so more fields are read quickly, giving higher sensitivity. Positive smears can be confirmed with a carbol fuchsin stain. It does not identify species.
Which yeast forms in tissue appear as a large cell with multiple buds attached, described as a 'ship's wheel' or 'mariner's wheel'?
Paracoccidioides produces a mother cell with many narrow-necked daughter buds around it. Blastomyces has a single broad-based bud.
Which selective medium is recommended to recover Burkholderia cepacia complex from respiratory specimens of patients with cystic fibrosis?
BCSA (or OFPBL agar) contains antibiotics that suppress P. aeruginosa and other CF flora, allowing slow-growing B. cepacia complex to be seen. CIN is for Yersinia, TCBS for Vibrio and Regan-Lowe for Bordetella.
A 30-year-old develops ascending flaccid paralysis two weeks after an episode of diarrhoea. Which enteric pathogen is most strongly linked to this complication?
Guillain-Barré syndrome follows C. jejuni infection because antibodies against its lipo-oligosaccharide cross-react with nerve gangliosides (molecular mimicry). Yersinia is linked with reactive arthritis rather than GBS.
Why is a thick blood film used together with a thin film for malaria diagnosis?
Thick films lyse red cells and examine more blood, so they are more sensitive. Thin films are methanol-fixed, keeping red cells intact for morphology and species identification.
Why must a fresh liquid stool for trophozoite detection be examined, or preserved, within about 30 minutes of passage?
Trophozoites in liquid stool are fragile and lose motility and morphology quickly. Formed stools contain mainly cysts and can wait longer.
In the formalin-ethyl acetate sedimentation method, the parasites are recovered from the:
Ethyl acetate extracts fats and debris into a top plug, while parasites sediment by centrifugation. The sediment is examined as wet mounts.
A round-to-oval egg about 60 µm with a thick, bumpy (mammillated) outer layer is seen in stool. It is:
Fertile Ascaris eggs have a thick mammillated coat and are often bile-stained brown. Trichuris eggs are barrel-shaped with polar plugs.
A thin-shelled oval egg about 60 × 40 µm containing an early cleaving embryo (4–8 cells) and a clear space under the shell is seen in stool. It is:
Hookworm eggs are thin-shelled with a cleaving embryo. Necator and Ancylostoma eggs cannot be separated by morphology.