MLT GlobeSCFHS exam preparation (Saudi Arabia) – page 38
749 practice MCQs for the SCFHS medical laboratory exam. Level: Intermediate.
Which IgG subclass has the longest hinge, the shortest half-life and the strongest ability to activate complement?
IgG3's long hinge makes it flexible and a strong C1q binder, but it has a half-life of only about 7 days. IgG1 is also a good activator but has a normal half-life.
In paroxysmal nocturnal haemoglobinuria, red cells lack CD59. The normal function of CD59 is to:
CD59 (membrane inhibitor of reactive lysis) prevents C9 from inserting and polymerising, so its loss allows MAC-mediated lysis. Accelerating convertase decay is the role of CD55 (DAF).
A 'virtual crossmatch' before organ offer is performed by:
A virtual crossmatch predicts compatibility by checking whether the recipient has antibodies against any HLA antigens in the donor's typing, without using donor cells. Mixing serum with donor lymphocytes is a physical crossmatch.
An E. coli is resistant to cefoxitin, cefotaxime and ceftazidime, remains susceptible to cefepime, and shows no zone increase with clavulanic acid. The most likely mechanism is:
AmpC enzymes hydrolyse cephamycins such as cefoxitin, are not inhibited by clavulanate and spare cefepime. ESBLs are inhibited by clavulanate and do not attack cefoxitin.
In broth microdilution, the trimethoprim-sulfamethoxazole wells show light 'trailing' growth over several concentrations. According to CLSI, the MIC is read as the lowest concentration giving:
Organisms can grow for a few generations before folate antagonists act, causing trailing. CLSI therefore reads sulfonamides and trimethoprim at ≥80% growth reduction, not at complete clearing.
A Klebsiella pneumoniae producing NDM carbapenemase tests resistant to ceftazidime-avibactam. The reason is that:
Avibactam inhibits class A (including KPC), class C and some class D enzymes, but not metallo-beta-lactamases such as NDM, VIM and IMP, which use zinc.
A farmer in Latin America has a swollen foot with draining sinuses that discharge small granules. The granules contain branching, partially acid-fast gram-positive filaments that grow aerobically. The most likely cause is:
Nocardia brasiliensis is a leading cause of actinomycetoma in the Americas. It is aerobic and partially acid-fast. Actinomyces is anaerobic and not acid-fast; Madurella is a fungus causing eumycetoma.
Sputum from a patient with cystic fibrosis grows tiny, non-pigmented, non-hemolytic colonies after 48–72 hours. They are catalase-positive cocci in clusters and weakly tube coagulase positive. The most likely explanation is:
S. aureus small-colony variants grow slowly, lose pigment and hemolysis, and often show weak or delayed coagulase. They are often thymidine- or hemin-dependent and persist in cystic fibrosis airways.
Stool from an HIV patient with chronic diarrhoea is stained with modified trichrome (chromotrope 2R). Tiny oval pinkish-red spores, 1–3 µm, some with a belt-like stripe, are seen. The organisms are:
Microsporidial spores are very small and stain pink-red with modified trichrome, often with a diagonal band. Cryptosporidium oocysts are larger (4–6 µm) and are shown with modified acid-fast stain.