MLT GlobeQCHP exam preparation (Qatar) – page 32
749 practice MCQs for the QCHP medical laboratory exam. Level: Intermediate.
Antimicrobial disk cartridges are taken from the freezer. Why should the closed container be allowed to reach room temperature before opening?
Opening a cold container lets moisture condense on the disks, and moisture degrades labile drugs such as beta-lactams. Disks are already active and sterile.
In disk diffusion testing, several distinct colonies grow inside an otherwise clear zone. The correct first action is to:
Colonies within a zone may be a contaminant or a resistant subpopulation. Purity is checked; if the culture is pure, the colony-free inner zone is measured.
A suspected Salmonella Typhi isolate agglutinates strongly in Vi antiserum but not in group D (O) antiserum. What should be done next?
The Vi capsular antigen can mask the O antigen and block O agglutination. Heating (boiling) the suspension removes Vi, after which group D O agglutination should appear.
Enteropathogenic E. coli (EPEC) causes infant diarrhoea mainly by:
EPEC attaches to small-bowel enterocytes with bundle-forming pili and intimin, destroying microvilli (attaching and effacing lesions). It does not produce Shiga toxin or LT/ST enterotoxins.
Empiric treatment of neonatal meningitis includes ampicillin because Listeria monocytogenes is intrinsically resistant to:
Listeria is intrinsically resistant to cephalosporins, so cephalosporin-only regimens fail. Ampicillin (often with gentamicin) is the treatment of choice.
A sentinel laboratory isolates a large, non-hemolytic, non-motile gram-positive rod from blood that looks like Bacillus anthracis. What should the laboratory do?
Sentinel laboratories must not try to fully identify possible bioterrorism agents. Further work is done in a biosafety cabinet and the isolate is referred to a Laboratory Response Network reference laboratory, with public health notified.
A man has weight loss, malabsorption and joint pain. Small-bowel biopsy shows foamy macrophages full of PAS-positive material, and an acid-fast stain is negative. PCR is most likely to detect:
Whipple disease is caused by Tropheryma whipplei, an actinobacterium seen as PAS-positive material in macrophages and confirmed by PCR. Mycobacterium avium complex also stains PAS positive but is acid-fast.
For culture of Corynebacterium diphtheriae in suspected respiratory diphtheria, the best specimen is:
Organisms are most numerous under the edge of the pseudomembrane, so a swab from there, together with a nasopharyngeal swab, gives the best yield. Blood cultures are rarely positive.
On Granada medium incubated anaerobically, beta-hemolytic Streptococcus agalactiae is recognized by:
Granada medium induces beta-hemolytic group B streptococci to produce an orange-red carotenoid pigment. Rare non-hemolytic strains do not form pigment and may be missed.
Guests vomit about 3 hours after eating cream pastries left at room temperature. Reheating the pastries would not have prevented illness because the cause is:
Staphylococcal enterotoxins are preformed in food and resist heating, so symptoms start within 1–6 hours. Illness from toxins made in the gut after ingestion has a longer incubation period.
A group B streptococcus is isolated from a pregnant woman with a severe penicillin allergy (high risk of anaphylaxis). What additional test should the laboratory perform?
For penicillin-allergic women at high anaphylaxis risk, clindamycin may be used for intrapartum prophylaxis, so clindamycin testing with detection of inducible resistance is required. GBS remain penicillin susceptible.
Routine susceptibility testing to penicillin is not needed for Streptococcus pyogenes because:
Group A streptococci remain uniformly susceptible to penicillin and other beta-lactams, so testing is not routinely needed. Macrolide or clindamycin testing is still useful for penicillin-allergic patients.
Nasopharyngeal specimens for Bordetella pertussis culture are plated on Regan-Lowe charcoal agar. After 3–5 days, typical colonies appear:
B. pertussis forms tiny, domed, shiny colonies resembling mercury drops or bisected pearls on Regan-Lowe or Bordet-Gengou agar. Black colonies with a brown halo describe C. diphtheriae on Tinsdale agar.
A rabbit hunter has a skin ulcer and tender axillary nodes. Tiny, faintly staining gram-negative coccobacilli grow after 3 days on chocolate agar and BCYE, but not on sheep blood agar. The most likely organism is:
Francisella tularensis causes ulceroglandular tularemia after handling rabbits and needs cysteine, so it grows on chocolate agar or BCYE. It is highly infectious, so work must be done in a biosafety cabinet.
Blood agar containing ampicillin is sometimes used to screen stool for Aeromonas because Aeromonas species:
Most Aeromonas species produce chromosomal beta-lactamases and are resistant to ampicillin, so they grow as beta-hemolytic, oxidase-positive colonies on ampicillin blood agar while much normal flora is inhibited.
A stool from a patient from the Philippines shows rounded eggs about 80 × 60 µm with a tiny, barely visible lateral knob. The most likely species is:
S. japonicum eggs are rounder and smaller than other schistosome eggs, with a minute lateral knob, and occur in East Asia. S. mansoni has a large lateral spine and S. haematobium a terminal spine.
A small (6–8 µm) oval cyst has four nuclei with large, blot-like karyosomes and no peripheral chromatin. It is most likely:
Endolimax nana cysts are small, with up to four nuclei that have large karyosomes and no peripheral chromatin. Entamoeba species show peripheral chromatin on the nuclear membrane.
During a norovirus outbreak on a hospital ward, staff are told to wash hands with soap and water rather than use alcohol hand rub. The main reason is that norovirus:
Non-enveloped viruses lack the lipid envelope that alcohol destroys, so alcohol rubs are less effective; soap and water physically remove the virus. Enveloped viruses are easily inactivated by alcohol.
A sample has an RBC count of 5.0 × 10^12/L and an MCV of 90 fL. What is the calculated hematocrit?
Hct (%) = RBC (× 10^12/L) × MCV (fL) ÷ 10 = 5.0 × 90 ÷ 10 = 45%.
Apart from hyposplenism, Howell-Jolly bodies are often seen in:
Disordered nuclear maturation in megaloblastic anemia leaves nuclear fragments, so Howell-Jolly bodies are common. They are not a feature of the other anemias listed.