MLT GlobeDOH exam preparation (Abu Dhabi) – page 32
700 practice MCQs for the DOH medical laboratory exam. Level: Basic to intermediate.
A fasting lipid profile shows total cholesterol 200 mg/dL, HDL cholesterol 50 mg/dL and triglycerides 150 mg/dL. Using the Friedewald equation, LDL cholesterol is:
LDL = TC − HDL − TG/5 = 200 − 50 − 30 = 120 mg/dL. Forgetting the VLDL term (TG/5) gives 150 mg/dL.
When a new lot of control material is introduced, the laboratory's own mean and SD should ideally be established from at least:
CLSI C24 recommends at least 20 results from 20 separate runs or days to capture day-to-day variation; ranges from only a few runs are unreliable. Values can be refined as more data accumulate.
On a Levey-Jennings chart, control values become widely scattered above and below the mean without a trend. Which cause is most likely?
Increased scatter reflects random error, such as intermittent bubbles in the pipetting system. Lamp deterioration or reagent evaporation usually causes trends, and a calibrator change causes a shift.
Two tests for the same disease are compared by receiver operating characteristic (ROC) curves. Test X has an area under the curve of 0.92 and test Y of 0.65. This means:
The area under the ROC curve summarises overall discrimination; 1.0 is perfect and 0.5 is no better than chance. A higher AUC does not guarantee better sensitivity or specificity at every single cut-off.
CSF from a patient with meningitis shows 25% eosinophils. The patient recently ate raw snails in Southeast Asia. The most likely cause is:
Eosinophilic meningitis suggests parasitic infection, especially the rat lungworm Angiostrongylus from raw snails or slugs. It is also seen with shunt reactions. Bacterial meningitis is neutrophilic.
Why is CSF for Gram stain concentrated by cytocentrifugation before staining?
Cytocentrifugation concentrates cells and bacteria onto a small area of the slide, detecting organisms at lower concentrations than a direct smear.
After a pelvic fracture, a patient has new ascites. Ascitic creatinine is much higher than serum creatinine. This most suggests:
Leakage of urine into the peritoneum makes ascitic creatinine (and urea) much higher than serum levels. Pancreatitis raises ascitic amylase instead.
In the qualitative fecal fat test, why is a second slide treated with acetic acid and heat before Sudan III staining?
Fatty acid salts (soaps) do not stain directly; acid and heat change them to free fatty acids that stain orange-red. The direct slide shows neutral fats only.
Some reagent strip manufacturers advise adding 0.005 to the specific gravity reading when the urine pH is 6.5 or higher. This is because:
The SG pad depends on release of hydrogen ions; in alkaline urine the indicator shift is blunted, so readings are falsely low. The correction compensates for this.
A urine shows many neutrophils on microscopy, but the leukocyte esterase pad is negative. The specific gravity is 1.035 and glucose is 4+. The most likely reason is:
High glucose, protein and specific gravity crenate white cells and slow esterase release, giving false-negative results. Oxidising agents cause false positives, and lysed neutrophils still release esterase.
Which condition favours the formation of urinary casts in the tubules?
Uromodulin precipitates more readily in acidic, concentrated urine with slow flow. Casts dissolve in dilute alkaline urine.
After a crush injury, a patient has red-brown urine with a positive blood pad and no RBCs. The sediment shows reddish-brown granular casts. These casts most likely contain:
Pigmented granular casts in rhabdomyolysis are formed from myoglobin, which is toxic to tubules. Bilirubin-stained casts are yellow and occur with jaundice.
The Sternheimer-Malbin stain used on urine sediment contains:
This supravital stain of crystal violet and safranin improves detail of nuclei, cells and casts. Oil Red O is used to identify lipids.
Forward and reverse ABO typing results do not agree. What is the first step?
Many discrepancies come from clerical or technical errors, so the tests are repeated first (with a new sample if a wrong-blood sample is suspected). The ABO group must not be reported until the discrepancy is resolved.
A woman with burning on urination has a urine that is positive for both leukocyte esterase and nitrite. This most suggests:
Leukocyte esterase indicates white cells (pyuria) and nitrite indicates nitrate-reducing bacteria; together they strongly suggest a bacterial urinary tract infection.
A patient with constant thirst and very large urine volumes has a persistently low specific gravity of about 1.002. This most suggests:
A persistently low specific gravity with polyuria shows that the kidneys cannot concentrate urine, as in diabetes insipidus. Dehydration and contrast media give a high specific gravity.
A patient with ascending weakness a few weeks after a diarrheal illness has CSF with markedly raised protein but a normal cell count. This pattern suggests:
High CSF protein with a normal white count (albuminocytologic dissociation) is typical of Guillain–Barré syndrome. Meningitis gives raised white cells.
Stool microscopy from a patient with bloody diarrhea and fever shows many neutrophils. This most suggests:
Fecal leukocytes indicate inflammation of the intestinal wall, as seen with invasive organisms such as Shigella, Salmonella and Campylobacter. Toxin-mediated and viral diarrheas usually lack them.
In tissue, Coccidioides immitis is seen as:
Coccidioides forms thick-walled spherules containing endospores in tissue, and arthroconidia in culture.
A dimorphic mould from an HIV patient from South-East Asia produces a diffusible red pigment at 25 °C. The most likely organism is:
T. marneffei is the only dimorphic 'Penicillium', produces a red diffusible pigment and causes disseminated infection in HIV patients in South-East Asia.