MLT GlobeMOH exam preparation (UAE) – page 8
700 practice MCQs for the MOH medical laboratory exam. Level: Basic to intermediate.
During the Gram stain, which cell-wall component traps the crystal violet-iodine complex in Gram-positive bacteria?
A thick, highly cross-linked peptidoglycan layer dehydrates with alcohol and retains the dye complex. Chitin and cellulose are fungal and plant wall materials.
Which spore-forming bacillus grows aerobically, not anaerobically?
Bacillus species are aerobic or facultative spore formers; Clostridium species are anaerobes.
A throat swab grows catalase-negative gram-positive cocci in chains with wide zones of complete haemolysis on blood agar; the isolate reacts with Lancefield group A antiserum. Identify it.
Group A beta-haemolytic streptococcus is S. pyogenes. S. agalactiae is group B, S. pneumoniae is alpha-haemolytic, and E. faecalis carries group D antigen and is usually non-haemolytic.
Which of these mycobacteria is an obligate human pathogen rather than an environmental saprophyte?
M. tuberculosis is a strict pathogen with humans as the reservoir. M. smegmatis, M. phlei and M. gordonae are environmental species usually regarded as contaminants.
Who first described 'animalcules' seen through his own single-lens microscopes in the 1670s?
Van Leeuwenhoek made the first recorded observations of bacteria and protozoa, earning him the title 'father of microbiology'. Pasteur and Koch worked two centuries later on germ theory and causation.
A sputum smear stained with auramine-rhodamine shows bright yellow bacilli under fluorescence microscopy. This stain is used to screen for:
Auramine-rhodamine binds mycolic acid and resists acid-alcohol, so acid-fast mycobacteria fluoresce. It is more sensitive than ZN for screening sputum.
After a correctly performed Gram stain, Escherichia coli cells appear:
Gram-negative cells lose crystal violet during decolourisation because of their thin peptidoglycan and outer membrane; they then take up the red safranin counterstain.
In a lymphocytic pleural effusion, which enzyme is measured to support a diagnosis of tuberculosis?
Adenosine deaminase, released by activated T lymphocytes, is raised in tuberculous effusions. Amylase suggests pancreatitis or oesophageal rupture.
Which genus is NOT in the order Enterobacterales?
Pseudomonas is an oxidase-positive, non-fermenting gram-negative rod. Enterobacterales are oxidase-negative glucose fermenters that reduce nitrate.
Which of these helical organisms is NOT a spirochaete?
H. pylori is a curved gram-negative rod with external sheathed flagella. Spirochaetes (Borrelia, Leptospira, Treponema) have periplasmic endoflagella.
Thin blood films are routinely fixed before Romanowsky (e.g. Giemsa) staining with:
Methanol fixes cells without lysing red cells or distorting morphology. Heat fixation is used for bacterial smears; formalin and mercuric chloride are histology fixatives.
Most cases of cholera are acquired by:
Vibrio cholerae is transmitted by the faecal–oral route, mainly contaminated water. It colonises the small intestine and is not spread by droplets or vectors.
A urine sample for culture cannot reach the laboratory within 2 hours and no boric acid container is available. Best action?
Refrigeration slows bacterial multiplication so colony counts stay meaningful for up to 24 hours. Room temperature allows overgrowth; acids kill organisms.
Standard gravity-displacement autoclaving of media and instruments uses:
Saturated steam at 121 °C (15 psi) for 15 minutes kills spores. 160 °C for 1–2 hours is dry heat in a hot-air oven.
Typical biochemical profile of Escherichia coli is:
Most E. coli are motile, produce indole from tryptophan and ferment lactose (pink on MacConkey). Klebsiella is non-motile; Proteus is lactose-negative.
Most bacteria multiply by:
A bacterial cell copies its chromosome and divides into two equal daughter cells. Endospores are resting forms, not a means of multiplication.
A loose, easily washed-off glycocalyx around a bacterium is termed:
A slime layer is loosely attached and unorganised; a capsule is firmly attached and well defined. Both are glycocalyx.
Clearing around colonies on DNA–methyl green agar is used to support identification of:
S. aureus produces heat-stable DNase (thermonuclease). Coagulase-negative staphylococci are usually DNase-negative.
Bacteria that colonise or infect the human body obtain carbon from organic compounds. They are therefore:
Human-associated bacteria are chemoheterotrophs using organic carbon. Autotrophs fix CO2 and chemolithotrophs oxidise inorganic compounds.
Prokaryotes lack mitochondria. Where does their electron transport chain operate?
Respiratory enzymes are embedded in the bacterial cytoplasmic membrane, which generates the proton motive force.