micro 6

Answer: B

​Rationale:

​A is incorrect: Bright red is retained by acid-fast organisms that absorb primary carbolfuchsin.

​B is correct: Non-acid-fast organisms stain blue when methylene blue is used as counterstain, or green when malachite green is used.

​C is incorrect: Purple is characteristic of Gram-positive organisms.

​D is incorrect: Yellow is not an endpoint for standard acid-fast counterstaining.

​E is incorrect: Black refers to tellurite reduction colonies.

In acid-fast staining procedures (such as Ziehl-Neelsen or Kinyoun), non-acid-fast organisms (NAFO) typically absorb the counterstain and appear:

A. Bright red

B. Blue or green

C. Purple

D. Yellow

E. Black

Answer: B
​Rationale:
​A is incorrect: Golden yellow is characteristic of S. aureus.
​B is correct: Potassium tellurite is reduced by Corynebacterium diphtheriae, imparting a characteristic gray-black color to colonies.
​C is incorrect: Metallic green sheen is seen in E. coli on EMB agar.
​D is incorrect: Pinkish-red is typical for lactose fermenters on MacConkey agar.
​E is incorrect: Bright blue indicates citrate positivity

On Cystine-Tellurite Blood Agar or Tinsdale medium, Corynebacterium diphtheriae produces colonies with what distinct color due to tellurite reduction?
A. Golden yellow
B. Gray to black
C. Metallic green
D. Pinkish-red
E. Bright blue

Answer: A
​Rationale:
​A is correct: Corynebacterium amycolatum is part of normal skin flora presenting with palisading/Chinese letter arrangements; often disregarded as a blood culture contaminant, it can cause sepsis and endocarditis.
​B is incorrect: S. epidermidis consists of gram-positive cocci in clusters.
​C is incorrect: Listeria displays small coccobacillary shapes.
​D is incorrect: Erysipelothrix forms thin, non-spore-forming rods.
​E is incorrect: Bacillus species are large spore-forming rods.

Which member of the normal skin and mucous membrane flora appears microscopic as pleomorphic gram-positive rods arranged in V-forms or "Chinese letters" and is an opportunistic agent of endocarditis?
A. Corynebacterium amycolatum
B. Staphylococcus epidermidis
C. Listeria monocytogenes
D. Erysipelothrix rhusiopathiae
E. Bacillus subtilis

​Answer: B
​Rationale:
​A is incorrect: Zinc sulfate flotation deforms operculated eggs and heavy helminth eggs.
​B is correct: Formalin-ethyl acetate sedimentation is the preferred standard concentration technique because it recovers all types of protozoan cysts, oocysts, and helminth eggs.
​C is incorrect: Harada-Mori is used for hatching hookworm and Strongyloides larvae.
​D is incorrect: Sheather's flotation is used primarily for Cryptosporidium oocysts.
​E is incorrect: Knott's technique concentrates blood microfilariae.

What is the preferred concentration technique for recovering parasite ova and cysts from stool specimens?

A. Zinc sulfate flotation

B. Formalin-ethyl acetate sedimentation

C. Harada-Mori filter paper culture

D. Sheather's sugar flotation

E. Knott's concentration technique

Answer: C
​Rationale:
​A is incorrect: S. epidermidis is not resistant.
​B is incorrect: S. saprophyticus is not susceptible.
​C is correct: Staphylococcus saprophyticus is resistant to 5ug novobiocin (zone < 16 mm), while Staphylococcus epidermidis is susceptible (zone ≥ 16 mm).
​D is incorrect: The susceptibility results are reversed.
​E is incorrect: Novobiocin differentiates coagulase-negative staphylococci.

How are Staphylococcus saprophyticus and Staphylococcus epidermidis differentiated using a 5 ug Novobiocin disk?
A. Both are resistant
B. Both are susceptible
C. S. saprophyticus is resistant; S. epidermidis is susceptible
D. S. saprophyticus is susceptible; S. epidermidis is resistant
E. Novobiocin testing is only used for Streptococci

Answer: B
​Rationale:
​A is incorrect: Gas bubble formation evaluates catalase.
​B is correct: In chromogenic cephalosporin testing, cleavage of the \beta-lactam ring produces a distinct color change (yellow to red), indicating \beta-lactamase activity.
​C is incorrect: Inhibition zones measure susceptibility.
​D is incorrect: Precipitate formation indicates lecithinase or double immunodiffusion.
​E is incorrect: Fluorescence detects specialized pigments like pyoverdin.

Detection of \beta-lactamase production using the chromogenic cephalosporin (nitrocefin) disk method relies on:
A. Formation of gas bubbles
B. Visible color change upon hydrolysis of the amide bond
C. Zone of growth inhibition
D. Precipitate formation in agar
E. Fluorescence under UV light

Answer: B
​Rationale:
​A is incorrect: Streptococcus and Shigella are DNase negative.
​B is correct: The classic DNase-positive bacterial triad ("S-M-S") includes Staphylococcus aureus, Moraxella catarrhalis, and Serratia marcescens.
​C is incorrect: Coliforms are DNase negative.
​D is incorrect: Non-fermenter group lacks diagnostic DNase activity across these species.
​E is incorrect: PPM group is defined by PAD positivity

Which triad of bacterial genera tests positive for Deoxyribonuclease (DNase) production?
A. Streptococcus, Micrococcus, Shigella
B. Staphylococcus aureus, Moraxella catarrhalis, Serratia marcescens
C. Escherichia, Klebsiella, Enterobacter
D. Pseudomonas, Burkholderia, Stenotrophomonas
E. Proteus, Providencia, Morganella

Answer: A
​Rationale:
​A is correct: Low concentration Bacitracin disks containing 0.02 to 0.04 units (Taxo A) inhibit Group A \beta-hemolytic Streptococci (S. pyogenes).
​B is incorrect: 5.0ug is the concentration used for optochin (Taxo P) or novobiocin disks.
​C is incorrect: 10 units is standard for general antimicrobial susceptibility disk formulations.
​D is incorrect: Incorrect range for Taxo A.
​E is incorrect: Incorrect unit value.

Group A \beta-hemolytic streptococci (Streptococcus pyogenes) are presumptive identified in the laboratory by susceptibility to which concentration of Bacitracin (Taxo A)?
A. 0.02 to 0.04 units
B. 5.0 units
C. 10.0 units
D. 1.0 to 2.0 units
E. 0.5 units

Answer: A
​Rationale:
​A is correct: A 6 mm disk containing 5ug optochin (ethylhydrocupreine hydrochloride) producing an inhibition zone of ≥ 14 mm identifies Streptococcus pneumoniae.
​B is incorrect: Represents resistance (viridans streptococci).
​C is incorrect: Indicates resistance.
​D is incorrect: Below susceptibility threshold.
​E is incorrect: Optochin causes growth inhibition, not antigen-antibody precipitation.

Susceptibility to 5ug Optochin (Taxo P) disk distinguishes Streptococcus pneumoniae from other \alpha-hemolytic streptococci. What zone of inhibition defines susceptibility?
A. ≥ 14 mm
B. Complete absence of zone (0 mm)
C. < 6 mm
D. Exactly 10 mm
E. Zone of precipitation

Answer: B
​Rationale:
​A is incorrect: S. pneumoniae is bile soluble.
​B is correct: Enterococcus species hydrolyze esculin in the presence of 40% bile, blackening Bile Esculin agar.
​C is incorrect: Bacitracin susceptibility isolates Group A streptococci.
​D is incorrect: Coagulase identifies S. aureus.
​E is incorrect: Enterococci are catalase/oxidase negative.

Which characteristic biochemical property is demonstrated by species belonging to the genus Enterococcus?
A. Bile solubility positive
B. Bile Esculin positive
C. Susceptible to Taxo A
D. Coagulase positive
E. Oxidase positive

Answer: B

​Rationale:

​A is incorrect: Peptidoglycan lysis occurs during lysozyme degradation.

​B is correct: The Quellung phenomenon occurs when homologous antiserum binds capsular polysaccharides, changing refractive index and producing apparent capsular swelling.

​C is incorrect: Swelling is immunologic, not physical osmosis.

​D is incorrect: Bile autolysis destroys cells without capsule swelling.

​E is incorrect: Quellung uses methylene blue/antiserum mixtures.

A positive Quellung reaction leads to microscopic "capsular swelling" caused by:

A. Enzymatic cleavage of cell wall peptidoglycan

B. Binding of specific anticapsular antibodies to capsular antigens

C. Water influx due to osmotic gradient changes

D. Autolysis of bacterial membranes by bile salts

E. Precipitated carbolfuchsin dye’’

Answer: A
​Rationale:
​A is correct: Serratia is distinguished from Klebsiella by failure to form gas from inositol, slow/delayed lactose fermentation, and rapid gelatinase/gelatin liquefaction.
​B is incorrect: Describes typical Klebsiella pneumoniae features.
​C is incorrect: Defines Proteeae tribe.
​D is incorrect: Serratia is H2S negative.
​E is incorrect: Both belong to Enterobacteriaceae (oxidase negative, fermentative).

Which combination of features differentiates Serratia species from Klebsiella species?

A. Rapid gelatin liquefaction, delayed lactose fermentation, and failure to produce gas from inositol

B. Non-motile, rapid lactose fermentation, gas production from inositol

C. Phenylalanine deaminase positive, urease positive

D. \text{H}_2\text{S} production, indole positive, citrate negative

E. Oxidase positive, non-fermentative

Answer: B
​Rationale:
​A is incorrect: Sheep blood agar provides X factor naturally in intact erythrocytes.
​B is correct: Staphylococcus colonies excrete V factor (nicotinamide adenine dinucleotide, NAD) as a metabolic byproduct, enabling V-factor-dependent Haemophilus to grow as satellite colonies.
​C is incorrect: Cysteine is required for Francisella.
​D is incorrect: Coagulase clots plasma.
​E is incorrect: Hemolysin releases internal red blood cell constituents, but active secretor release of V factor drives satellitism.

When satellite growth of Haemophilus occurs around feeder colonies of Staphylococcus on blood agar, the Staphylococcus supplies which essential growth factor?
A. X factor (Hemin) alone
B. V factor (NAD)
C. Cysteine
D. Coagulase enzyme
E. Hemolysin alone

Answer: B
​Rationale:
​A is incorrect: Campylobacter is identified via darting motility and microaerophilic growth.
​B is correct: Vibrio cholerae cells lyse in 0.5% sodium deoxycholate, releasing DNA that forms a viscous, mucinous string when lifted with an inoculating loop.
​C is incorrect: H. pylori is identified via rapid urease testing.
​D is incorrect: Identified via pyocyanin, grape-like odor, cetrimide growth.
​E is incorrect: Identified via serotyping and biochemical profiling.

A rapid diagnostic "string test" utilizing 0.5% sodium deoxycholate is used to presumptively identify:
A. Campylobacter jejuni
B. Vibrio cholerae
C. Helicobacter pylori
D. Pseudomonas aeruginosa
E. Salmonella typhi

​Answer: A
​Rationale:
​A is correct: Corynebacterium ulcerans produces colonies and pleomorphic rod morphology virtually identical to Corynebacterium diphtheriae on blood agar.
​B is incorrect: Listeria displays narrow zone beta-hemolysis and small rods.
​C is incorrect: Erysipelothrix forms small alpha or non-hemolytic colonies.
​D is incorrect: Arcanobacterium shows distinct wide-zone beta hemolysis.
​E is incorrect: Rhodococcus produces salmon-pink pigmented colonies.

Which organism shares colonial and microscopic morphology with Corynebacterium diphtheriae on Blood Agar Plate (BAP)?
A. Corynebacterium ulcerans
B. Listeria monocytogenes
C. Erysipelothrix rhusiopathiae
D. Arcanobacterium haemolyticum
E. Rhodococcus equi

Answer: B
​Rationale:
​A is incorrect: Both MTB and M. bovis are negative for heat-stable catalase at 68C.
​B is correct: M. tuberculosis is positive for both niacin accumulation and nitrate reduction, whereas M. bovis is negative for both.
​C is incorrect: Arylsulfatase isolates rapid growers (M. fortuitum-chelonae).
​D is incorrect: Non-standard primary differential combination.
​E is incorrect: MAC without crystal violet differentiates M. fortuitum-chelonae.

Mycobacterium tuberculosis (MTB) is best differentiated from Mycobacterium bovis using which biochemical test pair?
A. Catalase at 68C and Tween 80 hydrolysis
B. Niacin test and Nitrate reduction test
C. Arylsulfatase test and Pyrazinamidase test
D. Urease test and Tellurite reduction
E. MacConkey agar growth and Iron uptake

Answer: A
​Rationale:
​A is correct: Pulmonary anthrax caused by Bacillus anthracis is known as Woolsorter's disease due to occupational spore inhalation.
​B is incorrect: Black eschar (malignant pustule) describes cutaneous anthrax.
​C is incorrect: Undulant fever is brucellosis.
​D is incorrect: Lockjaw is tetanus (C. tetani).
​E is incorrect: Pontiac fever is non-pneumonic legionellosis.

Pulmonary anthrax, acquired by inhaling spores from contaminated animal hides or wool, is clinically referred to as:

A. Woolsorter's disease

B. Malignant pustule / Black Eschar

C. Undulant fever

D. Lockjaw

E. Pontiac fever

Answer: A

​Rationale:

​A is correct: Cutaneous anthrax lesions develop a central coal-black necrotic ulcer termed a black eschar.

​B is incorrect: Buboes occur in plague or LGV.

​C is incorrect: Erythema migrans is the bullseye rash of Lyme disease.

​D is incorrect: Hard chancres develop in primary syphilis.

​E is incorrect: Anthrax eschars are dry, coal-black, and non-purulent.

Cutaneous anthrax manifestations begin as a papule that progresses to a painless, necrotic lesion featuring a characteristic:

A. Black eschar

B. Bubo

C. Erythema migrans

D. Chancre

E. Eschar with green discharge

Answer: B

​Rationale:

​A is incorrect: Enteric clinical diagnostic media, not water standard media.

​B is correct: Standard confirmatory test media for coliform detection in water bacteriology include Endo agar, EMB agar, and Brilliant Green Lactose Bile (BGLB) broth.

​C is incorrect: Used in presumptive testing phases.

​D is incorrect: Fastidious pathogen media.

​E is incorrect: Selective non-coliform isolation media.

In microbiological water testing, which media are used as confirmatory culture media for coliforms?

A. MacConkey agar, Salmonella-Shigella agar, TCBS

B. Endo agar, Eosin-Methylene Blue (EMB) agar, Brilliant Green Lactose Broth (BGLB)

C. Nutrient broth, Lactose broth, Peptone water

D. Chocolate agar, Blood agar, Thayer-Martin

E. Cetrimide agar, BCYE, Fletcher's medium

Answer: D
​Rationale:
​A is incorrect: Statements II and III are also correct.
​B is incorrect: Statements I and III are also correct.
​C is incorrect: Statement III is also correct.
​D is correct: All three rules apply: binomial naming system, genus abbreviation (e.g., S. aureus), and derivation from Greek/Latin terms.
​E is incorrect: Statements I and II are also correct.

Which statements accurately reflect taxonomic naming rules for microorganisms?

I. Binomial nomenclature is utilized (Genus + species).

II. Genus name can be abbreviated using its initial capital letter.

III. Names are derived from Latin or Greek roots.

​A. I only

B. II only

C. I and II only

D. I, II, and III

E. III only

Answer: B
​Rationale:
​A is incorrect: Room temperature incubation used for room temperature fungal phases or motility.
​B is correct: Routine human bacterial pathogens are mesophilic, requiring an incubation temperature of 35 ± 2C.
​C is incorrect: Selective temperature for thermophilic Campylobacter.
​D is incorrect: Incubation for thermophilic spore-formers.
​E is incorrect: Refrigerator storage temperature.

What is the standard preferred temperature range (__C) for incubating routine bacteriological cultures?
A. 20 ± 2C
B. 35 ± 2C
C. 42 ± 2C
D. 55 ± 2C
E. 4 ± 2C

Answer: B

​Rationale:

​A is incorrect: Modified acid-fast stains fix oocysts.

​B is correct: Schaudinn's fixative preserves structural detail of intestinal protozoan trophozoites and cysts for permanent staining (e.g., Trichrome stain).

​C is incorrect: Thick blood smears fix microfilariae.

​D is incorrect: Concentrated wet mounts visualize eggs.

​E is incorrect: Lactophenol cotton blue fixes fungal elements.

Schaudinn's fixative solution is classically utilized in diagnostic parasitology to fix stool smears prior to staining for:

A. Coccidian oocysts

B. Protozoan trophozoites and cysts

C. Microfilariae

D. Nematode eggs

E. Fungal spores

Answer: B
​Rationale:
​A is incorrect: Incomplete architectural description.
​B is correct: Acanthamoeba castellanii cysts possess a double wall: an outer wrinkled ectocyst and an inner stellate/polygonal endocyst.
​C is incorrect: Describes Entamoeba histolytica cysts.
​D is incorrect: Describes Balantidium coli cysts.
​E is incorrect: Describes flagellate cyst structures.

Free-living amoeba Acanthamoeba castellanii exhibits double-walled cysts featuring which characteristic morphology?
A. Smooth inner wall, wrinkled outer wall
B. Outer wrinkled ectocyst wall and inner polygonal endocyst wall
C. Round double-layered wall with 4 nuclei
D. Single thick wall covered with cilia
E. Oval wall containing a prominent cytostome

Answer: B
​Rationale:
​A is incorrect: Common hosts are inverted.
​B is correct: Taenia saginata is the beef tapeworm; Taenia solium is the pork tapeworm.
​C is incorrect: Diphyllobothrium latum is the fish tapeworm.
​D is incorrect: Echinococcus granulosus is the dog tapeworm; Hymenolepis nana is the dwarf tapeworm.
​E is incorrect: Taenia asiatica is the Asian tapeworm.

Taenia saginata and Taenia solium are tapeworms acquired by consuming undercooked meat. What are their respective common names?
A. T. saginata = Pork tapeworm; T. solium = Beef tapeworm
B. T. saginata = Beef tapeworm; T. solium = Pork tapeworm
C. T. saginata = Fish tapeworm; T. solium = Beef tapeworm
D. T. saginata = Dog tapeworm; T. solium = Dwarf tapeworm
E. Both are called Asian tapeworms

Answer: D
​Rationale:
​A is incorrect: Matches II and III are also correct.
​B is incorrect: Matches I and III are also correct.
​C is incorrect: Match III is also correct.
​D is correct: All three associations are correct: L. donovani causes visceral disease (Kala-azar), L. tropica causes cutaneous lesions (Oriental sore/Delhi boil), and L. braziliensis causes mucocutaneous lesions (Espundia/Uta).

Match the Leishmania species with its corresponding disease entity:
I. Leishmania donovani \rightarrow Visceral Leishmaniasis (Kala-azar)
II. Leishmania tropica \rightarrow Cutaneous Leishmaniasis (Oriental sore)
III. Leishmania braziliensis \rightarrow Mucocutaneous Leishmaniasis (Espundia/Uta)

​A. I only
B. II only
C. I and II only
D. I, II, and III
E. III only

Answer: C

​Rationale:

​A is incorrect: RDTs are rapid screening tools used when microscopy is unavailable.

​B is incorrect: QBC is a fluorescent screening method.

​C is correct: Microscopic examination of thick (for screening/detection) and thin (for species identification and parasitemia quantification) blood smears remains the gold standard.

​D is incorrect: ELISA detects past exposure, not active parasitemia.

​E is incorrect: PCR is a confirmation tool, not primary field microscopy.

What is considered the gold standard laboratory method recommended by the Department of Health (DOH) for detecting and identifying Plasmodium species?
A. Rapid Diagnostic Test (RDT) cassettes
B. Quantitative Buffy Coat (QBC) analysis
C. Thick and thin blood smear examination
D. Serological ELISA antibody testing
E. Real-time PCR assay alone

Answer: B
​Rationale:
​A is incorrect: H. influenzae requires both X and V factors.
​B is correct: Haemophilus parainfluenzae is upper respiratory normal flora that requires V factor only (growth around V disk, no growth around X disk) and is non-hemolytic.
​C is incorrect: H. haemolyticus is \beta-hemolytic on blood agar.
​D is incorrect: H. ducreyi requires X factor only.
​E is incorrect: H. aphrophilus does not require X or V factor.

An organism isolated from the upper respiratory tract forms medium-to-large smooth colonies on Chocolate agar, fails to show hemolysis on Blood agar, and requires V factor alone for growth. This organism is:
A. Haemophilus influenzae
B. Haemophilus parainfluenzae
C. Haemophilus haemolyticus
D. Haemophilus ducreyi
E. Haemophilus aphrophilus

Answer: B
​Rationale:
​A is incorrect: The elementary body is the infectious, extracellular, metabolically inactive form.
​B is correct: The reticulate body is the non-infectious, intracellular, metabolically active replicative stage of Chlamydia.
​C is incorrect: Inclusion body refers to the intracellular vacuole containing both forms.
​D is incorrect: Trophozoite refers to protozoan feeding stages.
​E is incorrect: Spore refers to bacterial or fungal reproductive units.

What is the intracellular replicative form of Chlamydia species within host cell vacuoles?
A. Elementary body
B. Reticulate body
C. Inclusion body
D. Trophozoite
E. Spore

Answer: B
​Rationale:
​A is incorrect: S. pyogenes is PYR positive and bacitracin susceptible.
​B is correct: Group B Streptococcus (S. agalactiae) secretes extracellular CAMP factor that acts synergistically with staphylococcal \beta-lysin to produce arrowhead hemolysis on sheep blood agar.
​C is incorrect: S. pneumoniae is \alpha-hemolytic and optochin susceptible.
​D is incorrect: E. faecalis is bile esculin and 6.5% NaCl positive.
​E is incorrect: Viridans group streptococci are CAMP negative.

The CAMP test displays an enhanced arrowhead zone of hemolysis at the intersection of the test organism and a \beta-lysin-producing strain of Staphylococcus aureus. This synergestic hemolysis is characteristic of:
A. Group A Streptococcus (S. pyogenes)
B. Group B Streptococcus (S. agalactiae)
C. Streptococcus pneumoniae
D. Enterococcus faecalis
E. Viridans streptococci

Answer: B
​Rationale:
​A is incorrect: Only hookworm larvae characteristically settle at the bottom.
​B is correct: In Harada-Mori culture, Strongyloides filariform larvae migrate/float upward into fluid, while Hookworm filariform larvae settle at the bottom of the tube.
​C is incorrect: Hookworm larvae migrate downward.
​D is incorrect: The behavioral movement patterns are reversed.
​E is incorrect: Larvae actively migrate away from the feces on the paper strip.

In the Harada-Mori filter paper culture technique for recovering nematode larvae, what pattern of larval distribution occurs?
A. Both hookworm and Strongyloides larvae settle at the bottom
B. Strongyloides filariform larvae float/swim up; Hookworm filariform larvae settle at the bottom
C. Both hookworm and Strongyloides larvae float at the top
D. Hookworm larvae float; Strongyloides larvae settle
E. Larvae remain trapped in the center of the paper strip

Answer: A
​Rationale:
​A is correct: Congenital infection with Toxoplasma gondii causes classic ocular chorioretinitis, hydrocephalus, and cerebral calcifications.
​B is incorrect: T. trichiura causes rectal prolapse and whipworm infection.
​C is incorrect: E. histolytica causes amebic dysentery and liver abscesses.
​D is incorrect: G. lamblia causes malabsorptive diarrhea.
​E is incorrect: C. parvum causes self-limiting diarrhea in immunocompetent hosts.

Which parasite is an important cause of chorioretinitis and congenital CNS damage following transplacental transmission?
A. Toxoplasma gondii
B. Trichuris trichiura
C. Entamoeba histolytica
D. Giardia lamblia
E. Cryptosporidium parvum

Answer: A
​Rationale:
​A is correct: Trichuris trichiura attaches to cecal mucosa, causing mucosal breaks that facilitate tissue invasion by Entamoeba histolytica.
​B is incorrect: Giardia attaches non-invasively to duodenal mucosa.
​C is incorrect: Balantidium coli invades independently via hyaluronidase production.
​D is incorrect: Ascaris remains lumenal.
​E is incorrect: Strongyloides penetrates intact skin or intestinal wall.

Intestinal mucosal damage caused by heavy Trichuris trichiura (whipworm) infection provides a favorable portal of entry for secondary invasion by:
A. Entamoeba histolytica
B. Giardia lamblia
C. Balantidium coli
D. Ascaris lumbricoides
E. Strongyloides stercoralis

Answer: A
​Rationale:
​A is correct: Rhizopus is a ubiquitous, rapidly growing zygomycete mold commonly contaminating bread, commercially prepared foods, and laboratory media plates.
​B is incorrect: Common contaminant but Rhizopus represents the classic food contaminant noted here.
​C is incorrect: Common environmental mold but structurally distinct.
​D is incorrect: Common plant pathogen and corneal infective mold.
​E is incorrect: Dematiaceous mold contaminant.

Which zygomycete mold is widely recognized as a common airborne laboratory contaminant of commercially prepared food and culture media?
A. Rhizopus spp.
B. Aspergillus niger
C. Penicillium spp.
D. Fusarium spp.
E. Curvularia spp.

​Answer: B
​Rationale:
​A is incorrect: Enteric test battery.
​B is correct: Bordetella bronchiseptica reduces nitrate to nitrite and is rapidly urease positive, whereas Alkaligenes faecalis is nitrite negative and urease negative.
​C is incorrect: Both non-fermenters are catalase/oxidase positive.
​D is incorrect: Non-discriminatory test pair.
​E is incorrect: Used for Serratia identification.

Which biochemical test pair differentiates Bordetella bronchiseptica (positive for both) from Alkaligenes faecalis (negative for both)?
A. Indole and Methyl Red tests
B. Nitrite reduction and Urease tests
C. Catalase and Oxidase tests
D. Decarboxylase and Citrate tests
E. DNase and Gelatinase tests

Answer: B
​Rationale:
​A is incorrect: Staphylococcus lugdunensis is PYR positive.
​B is correct: Staphylococcus aureus is characteristically Coagulase positive and PYRase negative.
​C is incorrect: Characterizes Enterococcus or S. lugdunensis.
​D is incorrect: Characterizes S. epidermidis.
​E is incorrect: Incorrect enzymatic profile.

What is the expected key enzymatic profile of Staphylococcus aureus regarding Coagulase and Pyrrolidonyl Arylamidase (PYRase)?
A. Coagulase (+), PYRase (+)
B. Coagulase (+), PYRase (-)
C. Coagulase (-), PYRase (+)
D. Coagulase (-), PYRase (-)
E. Coagulase variable, PYRase (+)

Answer: A

​Rationale:

​A is correct: Inactivation or poisoning of the palladium catalyst pellets (by moisture or hydrogen sulfide) is the leading cause of system failure in anaerobic gas jars.

​B is incorrect: Indicator failure reveals oxygen presence but does not prevent reaction initiation.

​C is incorrect: Affects gas flow distribution secondary to catalyst failure.

​D is incorrect: Chemical packs are pre-formulated.

​E is incorrect: Mechanical leak rather than internal chemical system breakdown.

The most common cause of failure in an anaerobic GasPak jar system is:

A. Inactivated palladium catalyst

B. Expired methylene blue indicator strips

C. Overfilling with too many culture plates

D. Insufficient water added to packet

E. Cracked polycarbonate jar cover

Answer: B
​Rationale:
​A is incorrect: These produce Acid/Acid (\text{A/A}) reactions due to sucrose/lactose fermentation.
​B is correct: Plesiomonas, Salmonella, and Proteus ferment glucose only, yielding an Alkaline slant / Acid butt (\text{K/A}) profile on TSI.
​C is incorrect: Non-fermenters produce Alkaline/Alkaline (\text{K/K}) reactions.
​D is incorrect: These ferment sucrose to yield \text{A/A} TSI profiles.
​E is incorrect: Gram-positive organisms are not routinely inoculated onto TSI.

Which group of microorganisms yields an Alkaline slant / Acid butt (\text{K/A}) reaction on Triple Sugar Iron (TSI) agar?
A. Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae
B. Plesiomonas shigelloides, Salmonella spp., Proteus spp.
C. Pseudomonas aeruginosa, Acinetobacter baumannii
D. Vibrio cholerae, Yersinia enterocolitica
E. Streptococcus pyogenes, Staphylococcus aureus

Answer: A
​Rationale:
​A is correct: Legionella stains weakly with standard Gram stain; replacing safranin with 0.1% basic fuchsin (or extended carbolfuchsin) enhances visualization of thin gram-negative bacilli.
​B is incorrect: Methylene blue is used in diphtheria or acid-fast staining.
​C is incorrect: Malachite green counterstains spores or acid-fast preparations.
​D is incorrect: Primary Gram stain dye.
​E is incorrect: Fluorescent dye requiring UV microscopy.

When staining Legionella pneumophila in direct tissue specimens or smears, what counterstain is substituted for safranin to improve contrast and visualization?
A. Carbolfuchsin or Basic Fuchsin (0.1%)
B. Methylene blue
C. Malachite green
D. Crystal violet
E. Acridine orange