MLT GlobeDHA exam preparation (Dubai) – page 7
700 practice MCQs for the DHA medical laboratory exam. Level: Basic to intermediate.
Which transfusion complication usually appears weeks to months after transfusion?
Viral hepatitis has an incubation period of weeks to months. Septic reactions, urticaria and TACO appear during or within hours of transfusion.
Which anticoagulant is NOT used in blood-collection bags for transfusion?
EDTA chelates calcium irreversibly and harms platelet and red cell viability; transfusion bags use citrate-based solutions.
Which ABO/RhD type is the classic universal recipient for red cells?
AB D-positive recipients have no anti-A, anti-B or reason to form anti-D, so can receive red cells of any ABO and RhD type.
Gram-positive cocci from a pus sample bubble vigorously when mixed with hydrogen peroxide. This result separates:
Staphylococci produce catalase, releasing oxygen bubbles from H2O2; streptococci and enterococci are catalase-negative. Neisseria and Moraxella are both catalase-positive.
In the Gram stain, Gram's iodine forms a complex with crystal violet that resists decolourisation in Gram-positive cells. Iodine therefore acts as a:
A mordant fixes a dye in the cell; the crystal violet-iodine complex is trapped by thick peptidoglycan. Safranin is the counterstain.
A yeast incubated in serum at 37 °C for 2-3 hours produces short hyphal outgrowths without constriction at the base. This identifies:
A positive germ tube test is the rapid presumptive identification of C. albicans (and C. dubliniensis). C. glabrata and Cryptococcus do not form germ tubes.
Panton-Valentine toxin of S. aureus forms pores that kill neutrophils and macrophages. This class of exotoxin is called:
Leucocidins are exotoxins that destroy white blood cells. Enterotoxins act on the gut, neurotoxins on nerves; antitoxins are antibodies that neutralise toxins.
Helicobacter pylori colonises the stomach by producing urease, which generates ammonia around the organism. This allows it to survive in:
Urease converts urea to ammonia and CO2, buffering the organism's microenvironment so it survives gastric acid; this is also the basis of urea breath and rapid urease tests.
A disinfectant that kills bacteria, rather than merely stopping their growth, is described as:
Bactericidal agents cause irreversible cell death; bacteriostatic agents inhibit multiplication, which resumes when the agent is removed.
Which medium is recommended by CLSI and EUCAST for routine disc diffusion susceptibility testing of rapidly growing aerobes?
Mueller-Hinton agar is standardised for reproducibility, supports most pathogens and has low levels of sulphonamide/trimethoprim inhibitors. Nutrient agar is not standardised for AST.
A man has purulent urethral discharge. Neisseria gonorrhoeae most commonly gains entry to the body through which route?
Gonococci are sexually transmitted and attach to columnar epithelium of the urethra and cervix; pharyngeal and rectal sites follow specific exposures.
What structural feature makes Mycobacterium and Nocardia acid-fast?
Long-chain mycolic acids make the wall waxy, retaining carbol fuchsin despite acid-alcohol. Capsules and LPS do not confer acid-fastness.
Several neonates in a nursery develop S. aureus sepsis. Screening staff for carriage should focus on which body site?
The anterior nose is the main reservoir of S. aureus in healthy carriers, and nasal carriers often spread it via hands.
A 30-year-old woman has had watery, non-bloody diarrhoea for 14 hours. Which organism is least likely to be responsible?
S. pyogenes causes pharyngitis and skin infections, not enteritis. The others are recognised enteric pathogens, though Shigella often becomes bloody.
Most human pathogens, which grow well at body temperature, are classified as:
Mesophiles grow best between about 25 and 40 °C, including 37 °C, which is why clinical cultures are incubated at 35-37 °C.
The blue-green colour seen in pus and culture media from Pseudomonas aeruginosa infections is mainly due to which pigment?
Pyocyanin (blue) together with pyoverdine (yellow-green fluorescent) gives the typical green colour; pyocyanin is specific to P. aeruginosa. Prodigiosin is the red pigment of Serratia.
Which immunoglobulin circulates as a pentamer of five monomers joined by a J chain?
Serum IgM is pentameric (10 antigen-binding sites), making it an efficient agglutinator and complement activator. Secretory IgA is dimeric; IgG and IgE are monomers.
On a urine dipstick, which pad most specifically suggests the presence of Enterobacterales such as E. coli?
Gram-negative uropathogens reduce dietary nitrate to nitrite. Leukocyte esterase indicates pyuria; protein and glucose are not infection markers.
For which organism does a routine Gram stain give a reliable, characteristic result?
E. coli stains clearly as a Gram-negative rod. Mycobacteria resist Gram reagents, T. pallidum is too thin, and Mycoplasma lacks a wall.
A tease mount of a mould in lactophenol cotton blue is examined. This preparation is used to identify:
Lactophenol kills and preserves the fungus while cotton blue stains chitin in hyphae and spores, showing structures for identification.