MLT GlobeSCFHS exam preparation (Saudi Arabia) – page 33
749 practice MCQs for the SCFHS medical laboratory exam. Level: Intermediate.
Modified Thayer-Martin agar contains vancomycin, colistin, nystatin and trimethoprim. Trimethoprim is added to inhibit:
Trimethoprim lactate inhibits the swarming of Proteus. Vancomycin inhibits gram-positive bacteria, colistin inhibits most gram-negative rods and commensal Neisseria, and nystatin inhibits yeasts.
A skin flap becomes infected after medicinal leech therapy. The wound grows a beta-hemolytic, oxidase-positive, glucose-fermenting gram-negative rod. The most likely organism is:
Aeromonas species live in the leech gut and are a known cause of infection after leech therapy. They are oxidase positive, beta-hemolytic fermenters; P. aeruginosa does not ferment glucose.
A trichrome-stained stool shows round forms 6–20 µm, each with a large central body that pushes a thin rim of cytoplasm and small nuclei to the edge. The organism is:
The 'central body' form with peripheral nuclei is typical of Blastocystis. Iodamoeba cysts have a glycogen vacuole but only a single nucleus with a large karyosome.
Hookworm eggs are found in a stool concentrate. How should the species be reported?
Eggs of Necator americanus and Ancylostoma duodenale cannot be told apart by microscopy, so they are reported as 'hookworm eggs'. Species identification needs adult worms, cultured larvae or molecular tests.
The urine cytology of a kidney transplant recipient with rising creatinine shows epithelial cells with enlarged nuclei containing glassy inclusions ('decoy cells'). Which test is best next?
Decoy cells suggest BK polyomavirus reactivation; plasma BK viral load helps identify BK nephropathy, which is confirmed by biopsy. Bacterial culture does not detect it.
In severe anemia, large bluish-grey polychromatic red cells on the smear mainly represent:
Under strong EPO drive, the marrow releases young reticulocytes early (shift or stress reticulocytes). They still contain RNA, which makes them larger and bluish-grey on Wright stain.
In a patient with massive splenomegaly, the platelet count is low mainly because of:
Normally about one-third of platelets are held in the spleen. An enlarged spleen holds a much larger share, lowering the circulating count (hypersplenism).
Ristocetin-induced platelet aggregation is absent, but becomes normal when normal plasma is added. This pattern points to:
Ristocetin aggregation needs plasma von Willebrand factor and platelet GP Ib. Correction by normal plasma shows the missing part is plasma VWF. In Bernard-Soulier syndrome the defect is in the platelet and is not corrected.
The 'hair-on-end' appearance of the skull on X-ray in untreated beta-thalassemia major is caused by:
Ineffective erythropoiesis drives huge marrow expansion, which widens the skull bones and gives the hair-on-end pattern. Regular transfusion prevents it.
Giemsa stain for malaria films is buffered to about pH 7.2 mainly to:
At about pH 7.2, parasite chromatin, cytoplasm and red cell stippling such as Schüffner dots stain well. A more acid buffer may not show the dots.
Which blood group system, like ABO, has carbohydrate antigens?
Lewis antigens are carbohydrates made by fucosyltransferases and adsorbed onto red cells from plasma. Kell, Kidd and Duffy antigens are carried on proteins.
Cryoprecipitate is prepared by:
When FFP is thawed slowly in the cold, fibrinogen, factor VIII, VWF and factor XIII remain as a precipitate. This is collected and refrozen as cryoprecipitate.
Serious complications of an acute hemolytic transfusion reaction include:
Complement activation and cytokine release in acute intravascular hemolysis can cause shock, DIC and acute kidney injury.
Rh immune globulin is thought to prevent anti-D formation mainly by:
Passive anti-D coats fetal D-positive cells that entered the mother's blood, leading to their removal before she makes her own anti-D. The exact mechanism is not fully proven, but antigen clearance is the accepted textbook explanation.
In the Kleihauer-Betke acid-elution test, fetal red cells appear:
HbF resists acid elution and stays in the fetal cells, which stain with eosin. Adult HbA is washed out, leaving pale ghost cells.
A jaundiced patient has total bilirubin 4.0 mg/dL (68 µmol/L) and direct bilirubin 3.2 mg/dL (55 µmol/L). The hyperbilirubinemia is mainly:
Direct (conjugated) bilirubin makes up 80% of the total, so this is a conjugated hyperbilirubinemia, which points to hepatic or post-hepatic (obstructive) causes rather than hemolysis.
Blood is drawn from the same arm as a running 5% dextrose infusion. Which result is most likely to be falsely increased?
Contamination with dextrose infusion fluid greatly increases glucose and dilutes other analytes. Samples should be drawn from the opposite arm or below the infusion site.
PSA was undetectable after radical prostatectomy but is now rising on repeat tests. This most suggests:
After the whole prostate is removed, PSA should stay undetectable. A confirmed rising PSA indicates residual or recurrent prostate cancer (biochemical recurrence).
A patient has repeated severe pyogenic bacterial infections. Deficiency of which complement component would cause the most serious problems with opsonisation?
C3 is central to all complement pathways, and C3b is the main complement opsonin, so C3 deficiency causes severe recurrent pyogenic infections. C5–C9 deficiency mainly predisposes to Neisseria infections.
Which is an example of artificially acquired passive immunity?
Giving ready-made antibody by injection is artificial passive immunity. Vaccination is artificial active, recovery from infection is natural active, and placental IgG is natural passive.