MLT GlobeDHA exam preparation (Dubai) – page 13
700 practice MCQs for the DHA medical laboratory exam. Level: Basic to intermediate.
A mass spectrometer identifies and quantifies ions according to their:
Mass spectrometers ionize molecules and separate the ions by mass-to-charge ratio (m/z). Isoelectric point is the basis of isoelectric focusing, not mass spectrometry.
How many millilitres of a 10% stock solution are needed to prepare 200 mL of a 2% solution?
Using C1V1 = C2V2: 10% × V1 = 2% × 200 mL, so V1 = 40 mL, made up to 200 mL with diluent.
A stock standard contains 1000 mg/dL glucose. What volume of stock is needed to prepare 10 mL of a 50 mg/dL working standard?
V1 = (C2 × V2)/C1 = (50 × 10)/1000 = 0.5 mL, then dilute to 10 mL. 5.0 mL would give 500 mg/dL.
Routine handling of patient blood that may contain hepatitis B virus or HIV requires, at minimum:
Clinical labs handling human blood and body fluids operate at BSL-2 with standard precautions, PPE and biosafety cabinets for aerosol-producing procedures.
On the NFPA 704 hazard diamond, the blue section indicates the:
NFPA 704 uses blue for health, red for flammability, yellow for instability/reactivity and white for special hazards. Each is rated 0 (minimal) to 4 (severe).
In the PASS technique for using a fire extinguisher, the letter 'A' stands for:
PASS = Pull the pin, Aim at the base of the fire, Squeeze the handle, Sweep side to side. Aiming at the flames rather than the fuel source is ineffective.
Which antecubital vein is usually the first choice for routine venipuncture because it is large, well anchored and less painful?
The median cubital vein is well anchored and away from major nerves and arteries. The basilic vein lies near the brachial artery and median nerve, so it is the last choice.
Repeated control results are tightly grouped but far from the established target value. The method is best described as:
Close agreement between replicates means good precision; distance from the true value means poor accuracy (bias). Precision and accuracy are independent characteristics.
The main purpose of proficiency testing (external quality assessment) is to:
Proficiency testing sends samples of unknown value to many labs and grades each result against peers or target values, assessing accuracy externally. Daily precision is monitored by internal QC.
A reference interval is usually defined as the central 95% of results from healthy people. Therefore, among healthy individuals:
The central 95% excludes 2.5% at each end, so about 1 in 20 healthy people will have an 'abnormal' result. 32% applies to a ±1 SD range.
In most continuously monitored blood culture systems, routine bottles are incubated for how long before being reported as no growth?
Five days of incubation detects almost all clinically significant bacteria and yeasts, including HACEK organisms, in modern systems. Longer incubation is only needed in special situations.
Several people vomit 2 hours after eating fried rice that was kept warm for many hours. The most likely cause is a heat-stable emetic toxin from:
B. cereus spores survive cooking, and the emetic toxin (cereulide) forms in stored rice and causes vomiting within hours. C. perfringens causes diarrhoea after 8–16 hours.
Why is 10% potassium hydroxide (KOH) added to skin scrapings before microscopy for fungi?
KOH digests keratin and host cell debris, clearing the specimen so hyphae and yeasts stand out. It does not stain or fluoresce fungi; calcofluor white is the fluorescent reagent.
On cornmeal Tween 80 agar, which feature supports identification of Candida albicans?
Terminal chlamydospores on cornmeal agar with Tween 80 are characteristic of C. albicans (and C. dubliniensis). Arthroconidia suggest Trichosporon or Geotrichum.
A skin scraping from a patient with pale scaly patches on the trunk shows short curved hyphae and round yeasts ('spaghetti and meatballs'). Culture requires olive oil overlay. The organism is:
Malassezia causes pityriasis versicolor, shows the spaghetti-and-meatballs pattern, and most species require lipids for growth. Hortaea causes tinea nigra with dark palmar patches.
For which specimen is a Gram stain showing intracellular gram-negative diplococci in neutrophils considered sufficient for a presumptive diagnosis of gonorrhoea?
In symptomatic men, urethral Gram stain is highly sensitive and specific. Endocervical, rectal and pharyngeal sites contain commensal Neisseria and other gram-negative flora, so stain alone is not reliable there.
Giemsa stain for malaria films gives the best stippling (Schüffner dots) when the buffer pH is:
Giemsa buffered to pH 7.0–7.2 shows parasite chromatin, cytoplasm and stippling best. Acidic pH makes stippling poor and red cells too pink.
For suspected Loa loa infection, blood for microfilariae should be collected:
Loa loa shows diurnal periodicity, with peak numbers between about 10:00 and 14:00. Nocturnal collection is used for Wuchereria and Brugia.
For routine ova and parasite (O&P) examination, the recommended collection is:
Parasites are shed irregularly, so up to three specimens on different days improve detection. Barium and some medications interfere and should be avoided for 5–10 days.
Which is usually the first serologic marker detected after hepatitis B infection?
HBsAg appears 1–10 weeks after exposure, before symptoms and before any antibodies. Anti-HBs appears last, during recovery.