MLT GlobeASCP exam preparation (USA · MLS / MLT) – page 60
1200 practice MCQs for the ASCP medical laboratory exam. Level: Advanced.
A catalase-negative gram-positive coccus in pairs and tetrads is vancomycin resistant, PYR negative and LAP positive. It does not produce gas in MRS broth. The most likely genus is:
Pediococcus is intrinsically vancomycin resistant, LAP positive, PYR negative and makes no gas. Leuconostoc is also vancomycin resistant but produces gas and is LAP negative; aerococci are vancomycin susceptible.
Over one month, 9% of a laboratory's NALC-NaOH processed sputum cultures on solid media are overgrown by bacteria and fungi. The most likely cause is:
An acceptable contamination rate is about 2–5%. A higher rate suggests weak NaOH or too short exposure; a rate below 2% suggests over-harsh processing that also kills mycobacteria.
In the agar proportion method, a drug-free quadrant inoculated with a 1:100 dilution grows 60 colonies. The isoniazid quadrant, inoculated undiluted, grows 90 colonies. The isolate is:
The undiluted control equals 60 × 100 = 6000 CFU. Growth on drug is 90/6000 = 1.5%, which exceeds the 1% critical proportion, so the isolate is resistant.
An Xpert MTB/RIF Ultra result on sputum reads 'MTB detected trace; rifampicin resistance indeterminate'. This means:
'Trace' is the lowest positive category, detected by multi-copy targets; the rpoB region is not amplified enough to call resistance. The result is valid, not an internal-control failure.
A cryptococcal isolate turns canavanine-glycine-bromothymol blue (CGB) agar blue. This identifies the isolate as:
C. gattii resists canavanine and uses glycine, raising the pH so the indicator turns blue; C. neoformans leaves the medium yellow-green. The distinction matters for epidemiology and prognosis.
A patient with fever, rash and arthritis after a rat bite has negative blood cultures in bottles containing sodium polyanethol sulfonate (SPS). The best explanation is that:
Streptobacillus moniliformis, the main cause of rat-bite fever, is inhibited by SPS in most blood culture bottles. Culture needs SPS-free media enriched with serum or blood.
Which enzyme test separates Neisseria meningitidis (positive) from N. gonorrhoeae and N. lactamica?
In chromogenic enzyme substrate tests, gamma-glutamyl aminopeptidase is positive for N. meningitidis. N. gonorrhoeae is prolyl aminopeptidase positive and N. lactamica is beta-galactosidase positive.
A leg ulcer after seawater exposure grows an oxidase-positive non-fermenter. On TSI it gives alkaline slant/alkaline butt with blackening from H2S. The most likely organism is:
Shewanella is an unusual non-fermenter because it produces H2S in TSI. It is oxidase positive and found in marine environments. Salmonella and Proteus are oxidase-negative fermenters.
A man without a spleen develops sepsis with DIC after a dog bite. Blood cultures grow thin, fusiform gram-negative rods that need CO2, show gliding motility and are oxidase and catalase positive. The most likely organism is:
Capnocytophaga canimorsus from dog saliva causes fulminant sepsis in asplenic or alcoholic patients. It forms fusiform rods, needs CO2 and shows gliding motility; Pasteurella is a short coccobacillus.
Blood cultures from an elderly patient grow a curved, oxidase-positive gram-negative rod. It grows at 25 °C but not at 42 °C, is hippurate negative and is cephalothin susceptible. The most likely organism is:
C. fetus mainly causes bloodstream and systemic infection in elderly or immunocompromised patients. Unlike C. jejuni and C. coli, it grows at 25 °C, not at 42 °C, and is cephalothin susceptible.
Which transport medium should not be used for stool when Vibrio infection is suspected?
Glycerol is toxic to vibrios, so buffered glycerol saline is unsuitable, although it can be used for Shigella. Cary-Blair is the preferred transport medium for Vibrio.
A CSF cytospin from a patient with a ventricular shunt shows tight clusters of cells with round nuclei and abundant cytoplasm, looking uniform. They most likely are:
Choroid plexus and ependymal cells can appear in clusters after shunts or neurosurgery and have uniform nuclei. They must not be mistaken for malignant cells, which show pleomorphism.
A 65-year-old has rapidly progressive dementia and myoclonus. Which CSF test is the most specific for sporadic Creutzfeldt-Jakob disease?
RT-QuIC detects the seeding activity of the abnormal prion protein and has very high specificity. The 14-3-3 protein is a marker of rapid neuronal damage and is less specific.
A milky pleural effusion from a patient with long-standing tuberculous pleurisy has cholesterol 280 mg/dL (7.2 mmol/L), triglyceride 30 mg/dL (0.34 mmol/L) and cholesterol crystals. This is best classified as:
High cholesterol with low triglyceride and cholesterol crystals in a chronic effusion is pseudochylous. A true chylous effusion has triglyceride above 110 mg/dL and chylomicrons.
A heart-failure patient on diuretics has a pleural effusion that just meets an exudate criterion. Which finding best supports that it is really a transudate?
Diuretics concentrate pleural protein and LD; a serum–pleural albumin gradient above 1.2 g/dL (12 g/L) indicates a transudate. Very high LD and low pH point to an exudate such as empyema.
A man with diabetes has a semen volume of 0.3 mL with no sperm. A urine sample passed just after ejaculation contains many sperm. The most likely cause is:
In retrograde ejaculation, often due to diabetic neuropathy, semen passes into the bladder, so sperm appear in post-ejaculatory urine. With absent vas deferens or testicular failure, urine would not contain sperm.
In a pregnancy with maternal diabetes, the amniotic L/S ratio is 2.1. Which additional finding best confirms fetal lung maturity?
In diabetic pregnancies the L/S ratio can reach 2.0 while the lungs remain immature; phosphatidylglycerol appears later and confirms maturity. The fern test identifies amniotic fluid, not lung maturity.
Stool water from a patient with watery diarrhoea has sodium 30 mmol/L and potassium 40 mmol/L. Using an osmolality of 290 mOsm/kg, the fecal osmotic gap and likely diarrhoea type are:
Gap = 290 − 2(30 + 40) = 150 mOsm/kg. A large gap (above about 100–125) means unmeasured solutes such as poorly absorbed sugars or laxatives, indicating osmotic diarrhoea. Secretory diarrhoea has a gap below 50.
Refractile droplets in the urine of a nephrotic patient stain orange-red with Sudan III, but some droplets do not stain and show a 'Maltese cross' under polarised light. The non-staining droplets are:
Sudan III and Oil Red O stain triglycerides and neutral fats but not cholesterol, which is identified by Maltese cross patterns under polarised light. Using both methods detects all lipid types.
A patient with fever and flank pain has casts that appear granular under bright-field microscopy. A Gram stain of the sediment shows these casts are packed with bacilli. These are:
Bacterial casts look like granular casts but contain bacteria, confirmed by Gram stain; they indicate pyelonephritis. They are often accompanied by white cell casts.