MLT GlobeQCHP exam preparation (Qatar) – page 38
749 practice MCQs for the QCHP medical laboratory exam. Level: Intermediate.
An anaerobic, black-pigmented gram-negative rod is susceptible to vancomycin 5 µg but resistant to kanamycin 1 mg and colistin 10 µg special-potency disks. It is indole positive. It most likely belongs to the genus:
Porphyromonas is unusual among gram-negative anaerobes because it is vancomycin susceptible. Prevotella and Bacteroides are vancomycin resistant; Fusobacterium is kanamycin and colistin susceptible.
A Staphylococcus aureus isolate is resistant to cefoxitin by disk diffusion, but mecA PCR and a PBP2a latex test are both negative. The most likely explanation is:
mecC encodes a divergent PBP2a that causes cefoxitin resistance but is missed by mecA-specific PCR and many PBP2a latex tests. Penicillinase alone does not cause cefoxitin resistance.
A Staphylococcus aureus penicillin disk test gives a 30 mm zone (susceptible range), but the zone edge is sharp and cliff-like. What should be reported?
CLSI penicillin zone-edge test: a sharp, cliff-like edge means beta-lactamase production, so the isolate is reported penicillin resistant even with a large zone. A fuzzy, beach-like edge means beta-lactamase negative.
Daptomycin results should not be reported for MRSA isolated from sputum because daptomycin:
Pulmonary surfactant binds and inactivates daptomycin, so it fails in pneumonia even when the isolate tests susceptible. It remains useful for bacteremia and skin infections caused by MRSA.
A patient with endocarditis has blood cultures that grow a fastidious gram-negative rod. After 48 hours, colonies stick to the agar and show a star-shaped internal structure. It is catalase positive. The most likely organism is:
A. actinomycetemcomitans, a HACEK organism, forms adherent colonies with a star-shaped centre and is catalase positive. Cardiobacterium, Eikenella and Kingella are catalase negative.
Several neonates in an intensive care unit develop meningitis. The CSF grows an oxidase-positive, indole-positive non-fermenter with pale yellow colonies. It is resistant to colistin and carbapenems. The most likely organism is:
Elizabethkingia meningoseptica causes neonatal meningitis outbreaks. It is oxidase and indole positive, often yellowish, and intrinsically resistant to colistin and carbapenems. Acinetobacter and Stenotrophomonas are oxidase negative.
An isolate from an outbreak of cholera-like diarrhoea is biochemically Vibrio cholerae but does not agglutinate in O1 antiserum. What is the most appropriate next step?
Epidemic cholera is caused by serogroups O1 and O139. An O1-negative isolate from a cholera-like outbreak should be tested with O139 antiserum before being called non-O1/non-O139.
A night blood film from a patient from Malaysia shows sheathed microfilariae with two separate nuclei at the tip of the tail. The species is:
Brugia malayi microfilariae are sheathed and have two distinct terminal tail nuclei. W. bancrofti has no nuclei in the tail tip, and M. perstans is unsheathed.
A round egg about 75 µm with a six-hooked oncosphere is seen in stool. There are no polar filaments between the inner and outer membranes. It is most likely:
H. diminuta (rat tapeworm) eggs are larger than H. nana eggs and have polar thickenings without filaments. Taenia eggs have a thick striated shell.