MLT GlobeASCP exam preparation (USA · MLS / MLT) – page 31
1200 practice MCQs for the ASCP medical laboratory exam. Level: Advanced.
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.
Small oval yeasts, 2–4 µm, packed inside macrophages in a bone marrow smear from an HIV patient suggest:
Histoplasma capsulatum var. capsulatum appears as tiny intracellular yeasts within macrophages. Leishmania amastigotes look similar but have a kinetoplast.
Which dermatophyte genus produces many smooth, thin-walled, club-shaped macroconidia but rarely microconidia, and does NOT infect hair?
Epidermophyton floccosum forms smooth club-shaped macroconidia, often in clusters, with no microconidia, and infects skin and nails but not hair.
Microsporum canis is best recognized in culture by:
M. canis forms large spindle-shaped, rough, thick-walled macroconidia with six or more cells and a yellow colony reverse. Pencil-shaped smooth macroconidia suggest Trichophyton.
A fast-growing fluffy grey mold fills the Petri dish ('lid lifter') in 2 days. Sporangiophores arise directly above root-like rhizoids. The genus is:
Rhizopus has nodal rhizoids with sporangiophores arising opposite them. Mucor lacks rhizoids, and Lichtheimia has internodal rhizoids with pear-shaped sporangia.
Skin biopsy from a farm worker with chronic warty lesions of the leg shows brown, thick-walled, round cells with cross-walls ('copper pennies'). The diagnosis is:
Pigmented muriform (sclerotic) bodies in tissue define chromoblastomycosis, caused by agents such as Fonsecaea pedrosoi. Phaeohyphomycosis shows pigmented hyphae, and mycetoma shows grains.
A gram-negative diplococcus from a urethral culture gives immediate, strong bubbling when a drop of 30% hydrogen peroxide is added. This superoxol result is most typical of:
N. gonorrhoeae gives a strong (4+), immediate superoxol reaction. N. lactamica and N. cinerea give weak reactions, and Kingella is catalase negative. (Moraxella catarrhalis can also react strongly, so superoxol is only presumptive.)
An oxidase-positive gram-negative diplococcus from a middle-ear aspirate is asaccharolytic, DNase positive and butyrate esterase positive. Colonies slide intact across the agar when pushed. It is:
M. catarrhalis produces no acid from sugars, is DNase and butyrate esterase positive, and forms 'hockey puck' colonies. N. meningitidis produces acid from glucose and maltose.
A wound from a human bite grows an oxidase-positive, catalase-negative gram-negative rod that pits the agar and smells of bleach after 48 hours in CO2. It is most likely:
Eikenella corrodens, a HACEK member of oral flora, pits agar and has a bleach-like odour. Pasteurella and Capnocytophaga are linked to animal bites.
Buffered charcoal yeast extract (BCYE) agar supports Legionella mainly because it supplies:
Legionella needs L-cysteine and is helped by soluble iron; charcoal removes toxic oxygen radicals. It does not grow on standard blood agar, which lacks enough cysteine.
A patient has watery diarrhoea after eating raw shellfish. The stool grows green (sucrose-negative) colonies on TCBS agar. The isolate is oxidase positive and needs added NaCl for growth. The most likely organism is:
V. parahaemolyticus is halophilic, does not ferment sucrose (green colonies on TCBS) and causes seafood-associated gastroenteritis. V. cholerae ferments sucrose (yellow colonies) and grows without added salt.
An oxidase-positive, glucose-fermenting gram-negative rod is isolated from rice-water stool. Which result best supports Vibrio rather than Aeromonas?
Vibrio cells lyse in 0.5% sodium deoxycholate, releasing DNA that forms a mucoid string; Aeromonas is string-test negative. Both genera are oxidase positive and both may be haemolytic and grow on MacConkey.
Two curved, oxidase-positive gram-negative rods from stool both grow at 42 °C. Which test separates Campylobacter jejuni from C. coli?
C. jejuni hydrolyzes hippurate, while C. coli does not. Both species are oxidase and catalase positive, and nalidixic acid results are no longer reliable because of widespread quinolone resistance.
A patient with dyspepsia has a negative urea breath test. He has been taking a proton pump inhibitor daily for the past month. What is the most appropriate action?
Proton pump inhibitors suppress H. pylori urease activity and cause false-negative breath and stool antigen tests; they should be stopped about 2 weeks before testing. Serology stays positive after cure, so it cannot confirm current status.
A traveller from Malaysian Borneo has daily fevers and high parasitemia. Early forms look like P. falciparum and later forms look like P. malariae band forms. The most likely species is:
P. knowlesi, a zoonotic macaque malaria in Southeast Asia, has a 24-hour cycle, can reach high parasitemia, and mimics P. falciparum rings and P. malariae bands. PCR confirms it.
A bone marrow aspirate shows small oval organisms inside macrophages, each with a nucleus and a rod-shaped kinetoplast. The patient has fever, weight loss and splenomegaly. The diagnosis is:
Leishmania amastigotes are identified by a nucleus plus a bar-shaped kinetoplast. Histoplasma yeasts look similar but have no kinetoplast.
An Entamoeba cyst has 8 nuclei with eccentric karyosomes and splinter-like chromatoid bodies. It is:
Mature E. coli cysts have up to 8 nuclei and splinter-like chromatoid bodies. E. histolytica/dispar cysts have up to 4 nuclei and smooth, rounded chromatoid bars.
In a two-vial stool collection system, the vial containing 10% formalin is mainly used for:
Formalin preserves eggs, larvae and cysts well for concentration methods. The second vial (PVA or a zinc/SAF substitute) is used to prepare permanent stained smears.
A stool concentrate shows larvae with a short buccal cavity and a prominent genital primordium. The patient has eosinophilia and is starting corticosteroid therapy. The organism is:
Rhabditiform larvae of Strongyloides have a short buccal canal and a visible genital primordium; hookworm larvae have a long buccal canal. Steroids can trigger fatal hyperinfection, so detection is urgent.