MLT GlobeDHA exam preparation (Dubai) – page 25
700 practice MCQs for the DHA medical laboratory exam. Level: Basic to intermediate.
An Enterococcus faecalis from endocarditis shows no zone (6 mm) around a 120 µg gentamicin disk. This result means:
The high-content gentamicin disk screens for high-level aminoglycoside resistance. A 6 mm zone means the aminoglycoside will not act synergistically with ampicillin or vancomycin. The standard 10 µg disk is not used for this purpose.
When preparing a cumulative antibiogram according to CLSI M39, the laboratory should include:
M39 uses the first isolate per patient per period to avoid bias from repeat cultures of resistant organisms. It also recommends at least 30 isolates of a species for reliable percentages.
Escherichia coli forms yellow colonies on XLD agar because it:
E. coli ferments xylose, lactose and sucrose; the acid turns phenol red yellow. Lysine decarboxylation raises the pH and gives a red colour.
Two swarming, H2S-positive, urease-positive isolates differ in one test: isolate 1 is indole positive, isolate 2 is indole negative. Isolate 1 is most likely:
P. vulgaris is indole positive; P. mirabilis is indole negative. Morganella and Providencia do not swarm.
Which carbapenemase requires zinc at its active site?
NDM, VIM and IMP are Ambler class B metallo-beta-lactamases that require zinc. KPC and OXA-48 are serine enzymes; CTX-M is an ESBL.
Sputum from a renal transplant patient shows thin, beaded, branching gram-positive filaments. They stain red with a modified Kinyoun stain using 1% sulfuric acid but not with the standard acid-fast stain. The most likely genus is:
Nocardia is partially acid-fast: it keeps carbol fuchsin when a weak acid decolourizer is used. Actinomyces and Streptomyces are not acid-fast, and Mycobacterium resists strong acid-alcohol and does not branch.
A fish handler has a painful purple swelling on the finger. The culture grows a catalase-negative, non-motile gram-positive rod that produces H2S in the butt of a TSI slant. The most likely organism is:
Erysipelothrix causes erysipeloid in people who handle fish or meat and is the only common gram-positive rod that produces H2S in TSI. Listeria is catalase positive and H2S negative.
When plating a throat swab on sheep blood agar, the agar is stabbed with the loop. The main reason is to:
Streptolysin O is inactivated by oxygen, so hemolysis is strongest below the agar surface. Stabbing reveals subsurface beta-hemolysis of group A streptococci.
Colonies of Streptococcus pneumoniae often develop a sunken (umbilicated) centre after 24–48 hours. This is caused by:
The autolysin (LytA) lyses older cells in the colony centre, producing the 'draughtsman' appearance. The same enzyme is responsible for bile solubility.
Growth in a Mycobacteria Growth Indicator Tube (MGIT) is detected because:
MGIT contains a fluorescent compound in silicone that is quenched by oxygen; as mycobacteria consume oxygen, fluorescence increases and the instrument flags the tube. CO2 colorimetric detection is used by some blood culture systems.
Calcofluor white is useful for direct fungal detection because it:
Calcofluor white binds beta-linked polysaccharides such as chitin and cellulose in fungal walls and fluoresces blue-white or apple-green depending on filters. A clear halo describes India ink with Cryptococcus.
A yeast isolate from a patient in an ICU is resistant to fluconazole, is misidentified by some biochemical systems, and spreads between patients on surfaces. Which organism is of greatest concern?
Candida auris is often multidrug resistant, persists on surfaces, causes healthcare outbreaks and is misidentified by older biochemical methods. MALDI-TOF with an updated database or molecular methods is recommended.
Which method is recommended for reliable identification of Candida auris?
MALDI-TOF MS with a validated, updated library or DNA sequencing reliably identifies C. auris. It is germ tube negative and gives no characteristic colony on plain SDA, so these cannot identify it.
Which serum or CSF test is most sensitive and specific for diagnosing cryptococcal meningitis?
Cryptococcal antigen (CrAg) lateral flow assay detects capsular glucuronoxylomannan with very high sensitivity. India ink misses many cases, and Cryptococcus has little beta-D-glucan.
Which medium produces brown to black colonies of Cryptococcus neoformans because of phenoloxidase activity?
Niger seed (birdseed) agar contains caffeic acid, which C. neoformans phenoloxidase converts to melanin, giving brown colonies. Cornmeal agar is for morphology, and Mycosel contains cycloheximide, which inhibits Cryptococcus.
Mycosel (Mycobiotic) agar is unsuitable as the only medium for a CSF fungal culture because cycloheximide inhibits:
Cycloheximide inhibits saprophytic molds and several pathogens, including Cryptococcus, some Candida species and Aspergillus. Dermatophytes and dimorphic fungi tolerate it, so a medium without cycloheximide must also be inoculated.
Tissue from a patient with pulmonary disease shows large yeasts (8–15 µm) with thick double-contoured walls and single broad-based buds. The most likely agent is:
Broad-based single budding of large thick-walled yeasts is typical of Blastomyces. Histoplasma yeasts are small (2–4 µm) and intracellular; Paracoccidioides shows multiple narrow buds.
When tissue is submitted for suspected mucormycosis, the laboratory should mince it rather than grind it because:
Pauciseptate hyphae of Mucorales are easily destroyed by homogenization, which lowers culture recovery. Gentle mincing of tissue keeps hyphae viable.
A man has a painful, soft genital ulcer with tender inguinal lymph nodes. A Gram stain of the ulcer base shows small gram-negative coccobacilli in parallel chains ('school of fish'). The most likely organism is:
H. ducreyi causes chancroid, a painful ulcer, and shows chains of coccobacilli. Syphilitic chancres are painless, and K. granulomatis shows Donovan bodies inside macrophages.
A shepherd who drinks unpasteurized goat milk has undulating fever for weeks. After 4 days, blood cultures grow tiny gram-negative coccobacilli that are oxidase, catalase and urease positive. The most likely organism is:
Brucella is a slow-growing, tiny coccobacillus that is oxidase, catalase and urease positive; B. melitensis is linked to goat and sheep milk products and causes undulant fever. It is a major cause of laboratory-acquired infection, so work must move to a biosafety cabinet. Francisella is oxidase negative.