MEDICAL BACTERIOLOGY (MBMM3311) - MCQs | MCQ Quiz | OmpathStudy Kenya

Question 1

A gram-positive coccus isolated from a wound shows golden-yellow colonies on blood agar but is coagulase negative. The most likely organism is

Accepted Answer

Staphylococcus saprophyticus — S. saprophyticus can produce yellow pigment but is coagulase negative, unlike S. aureus which is coagulase positive. This organism is commonly associated with urinary tract infections in young women.

Question 2

A patient with "rice water" stool most likely has infection with

Accepted Answer

Vibrio cholerae — Rice water stool (watery with flecks of mucus resembling water in which rice has been washed) is characteristic of cholera caused by V. cholerae. The toxin causes massive fluid secretion leading to severe dehydration.

Question 3

Which organism can grow at refrigeration temperature (4°C)?

Accepted Answer

Listeria monocytogenes — Listeria is psychrophilic and can multiply at refrigeration temperatures (4°C), making it a significant food safety concern in refrigerated ready-to-eat foods. This unique ability distinguishes it from most other pathogenic bacteria.

Question 4

A 25-year-old sexually active female presents with urinary tract infection. The organism that is coagulase-negative and novobiocin resistant is

Accepted Answer

Staphylococcus saprophyticus — S. saprophyticus is novobiocin resistant and causes UTI in young sexually active women, being the second most common cause of UTI after E. coli. S. epidermidis is novobiocin sensitive, which helps differentiate these two coagulase-negative staphylococci.

Question 5

The "string of pearls" appearance on Loeffler serum medium is characteristic of

Accepted Answer

Bacillus anthracis — B. anthracis shows string of pearls appearance when grown in presence of penicillin on Loeffler medium. This occurs due to the formation of round bodies along the bacilli chain, creating a beaded appearance.

Question 6

A neonate develops meningitis. CSF culture grows gram-negative bacilli with K1 capsular antigen. The organism is

Accepted Answer

Escherichia coli — E. coli with K1 capsular antigen is the most common cause of neonatal meningitis. The K1 polysaccharide capsule helps the organism evade the immature neonatal immune system and cross the blood-brain barrier.

Question 7

"Medusa head" colonies on blood agar are produced by

Accepted Answer

Bacillus anthracis — B. anthracis produces characteristic medusa head or curled hair lock colonies due to chaining of bacilli. The colonies have irregular edges with comma-shaped projections resembling the snakes on Medusa's head.

Question 8

Which test differentiates Staphylococcus from Streptococcus?

Accepted Answer

Catalase test — Staphylococcus is catalase positive (produces bubbles when hydrogen peroxide is added) while Streptococcus is catalase negative. This is the primary test used to differentiate these two genera of gram-positive cocci.

Question 9

A patient develops pseudomembranous colitis after prolonged antibiotic therapy. The causative organism is

Accepted Answer

Clostridium difficile — C. difficile produces toxins A (enterotoxin) and B (cytotoxin) causing antibiotic-associated pseudomembranous colitis. The condition develops when normal gut flora is disrupted by antibiotics, allowing C. difficile overgrowth.

Question 10

"Chinese letter" or "V and L shaped" arrangement is seen in

Accepted Answer

Corynebacterium diphtheriae — C. diphtheriae shows characteristic angular arrangement resembling Chinese letters or cuneiform writing due to snapping division. The cells remain attached at angles after division, creating V or L shapes.

Question 11

A burn patient develops green-blue pus with a fruity odor. The organism is

Accepted Answer

Pseudomonas aeruginosa — Pseudomonas produces pyocyanin (blue-green pigment) and pyoverdine giving green-blue pus with a characteristic grape-like or fruity odor. This organism is a common opportunistic pathogen in burn wounds.

Question 12

The CAMP test is positive for

Accepted Answer

Streptococcus agalactiae — Group B Streptococcus (S. agalactiae) shows enhanced hemolysis (arrowhead-shaped zone) when grown near S. aureus on blood agar. This CAMP (Christie, Atkins, Munch-Petersen) test is used for definitive identification.

Question 13

Which organism produces black colonies on bismuth sulfite agar?

Accepted Answer

Salmonella typhi — S. typhi produces hydrogen sulfide (H2S) which reacts with bismuth sulfite to form black bismuth sulfide, producing characteristic black colonies with metallic sheen. This is used for selective isolation of S. typhi.

Question 14

A patient presents with flaccid paralysis and descending weakness. The organism that causes this by blocking acetylcholine release is

Accepted Answer

Clostridium botulinum — C. botulinum toxin blocks acetylcholine release at neuromuscular junctions causing flaccid paralysis with descending pattern (starting from cranial nerves), unlike tetanus which causes spastic paralysis with muscle rigidity.

Question 15

"Swarming" motility on blood agar is characteristic of

Accepted Answer

Proteus species — Proteus shows swarming motility creating concentric waves or ripples on agar due to highly active flagella and differentiation into elongated swarmer cells. This can be inhibited by increasing agar concentration or adding bile salts.

Question 16

A gram-negative coccus that ferments maltose but not glucose is

Accepted Answer

None of the above — This is a trick question - Neisseria species always ferment glucose first before maltose. N. gonorrhoeae ferments only glucose, while N. meningitidis ferments both glucose and maltose. No Neisseria ferments maltose without fermenting glucose.

Question 17

"Elek test" is used to detect toxigenicity of

Accepted Answer

Corynebacterium diphtheriae — Elek immunoprecipitation test detects diphtheria toxin production by C. diphtheriae strains. Lines of precipitation form between toxin-producing colonies and antitoxin-soaked filter paper strips on agar.

Question 18

Which organism produces "boxcar" shaped gram-positive rods?

Accepted Answer

Clostridium perfringens — C. perfringens appears as large, rectangular, boxcar-shaped or brick-shaped gram-positive rods. They are non-motile and characteristically lack spores when grown in tissue (though they form spores in culture).

Question 19

A patient develops hemolytic uremic syndrome (HUS) after eating undercooked hamburger. The organism is

Accepted Answer

Escherichia coli O157:H7 — Enterohemorrhagic E. coli (EHEC), particularly O157:H7, produces Shiga toxin causing hemorrhagic colitis that can progress to HUS with acute renal failure, thrombocytopenia, and microangiopathic hemolytic anemia. Transmitted via contaminated beef.

Question 20

"Drumstick" or "tennis racket" appearance of spores is seen in

Accepted Answer

Clostridium tetani — C. tetani has terminal round spores that give the bacillus a characteristic drumstick or tennis racket appearance. This terminal spore position distinguishes it from C. botulinum which has subterminal spores.

Question 21

The most common cause of acute bacterial meningitis in adults is

Accepted Answer

Streptococcus pneumoniae — S. pneumoniae (pneumococcus) is the leading cause of bacterial meningitis in adults and elderly, accounting for approximately 50% of cases. It also causes pneumonia, otitis media, and sinusitis.

Question 22

A stool sample shows gram-negative curved rods with "gull wing" or "S" shape. The organism is

Accepted Answer

Campylobacter jejuni — Campylobacter has characteristic S-shaped, gull wing, or spiral morphology and shows rapid darting corkscrew motility. It is a leading cause of bacterial gastroenteritis worldwide, transmitted through contaminated poultry.

Question 23

Which organism is urease positive and causes "alkaline tide" in urine?

Accepted Answer

Proteus mirabilis — Proteus produces urease enzyme that splits urea to ammonia and carbon dioxide, raising urine pH (alkaline tide). This promotes formation of struvite (magnesium ammonium phosphate) kidney stones and allows organism identification.

Question 24

"Nagler reaction" is positive for

Accepted Answer

Clostridium perfringens — Nagler test detects lecithinase (alpha toxin/phospholipase C) production by C. perfringens. The organism causes opacity on one half of egg yolk agar, while the other half (containing antitoxin) remains clear.

Question 25

A patient with infective endocarditis has blood culture showing catalase negative, gram-positive cocci resistant to optochin and not lysed by bile. The organism is

Accepted Answer

Enterococcus faecalis — Enterococcus is optochin resistant, bile insoluble, and catalase negative, distinguishing it from S. pneumoniae (optochin sensitive, bile soluble). Enterococci are common causes of subacute endocarditis and UTIs.

Question 26

"Quellung reaction" is used to identify capsule of

Accepted Answer

All of the above — Quellung reaction (capsular swelling test) can identify capsules of any encapsulated bacteria when mixed with specific anticapsular antisera. The capsule appears to swell under microscopy, allowing serotype identification.

Question 27

Which organism causes "woolsorter's disease"?

Accepted Answer

Bacillus anthracis — Woolsorter's disease is inhalational anthrax from B. anthracis spores in contaminated animal products (wool, hides, hair). It causes severe hemorrhagic mediastinitis with high mortality and was historically an occupational disease of wool handlers.

Question 28

A comma-shaped gram-negative rod showing "shooting star" motility in hanging drop is

Accepted Answer

Vibrio cholerae — V. cholerae shows characteristic rapid darting or shooting star motility in hanging drop preparation due to its single polar flagellum. This distinctive motility pattern aids in presumptive identification.

Question 29

"Widal test" detects antibodies against

Accepted Answer

Salmonella typhi — Widal test detects agglutinating antibodies against O (somatic) and H (flagellar) antigens of Salmonella typhi in typhoid fever. A four-fold rise in titer is significant, though the test has limitations in endemic areas.

Question 30

Which organism produces "currant jelly" sputum?

Accepted Answer

Klebsiella pneumoniae — Klebsiella pneumonia produces thick, bloody, mucoid sputum resembling currant jelly due to extensive tissue necrosis and the organism's large polysaccharide capsule. This destructive pneumonia often affects alcoholics and diabetics.

Question 31

"McFadyean reaction" showing pink-staining capsule is diagnostic for

Accepted Answer

Bacillus anthracis — McFadyean polychrome methylene blue staining shows characteristic pink/purple capsule around blue B. anthracis bacilli in blood or tissue smears. This is used for rapid diagnosis of anthrax in animals.

Question 32

A patient develops gas gangrene after a crushing injury. The organism most likely responsible is

Accepted Answer

Clostridium perfringens — C. perfringens produces alpha toxin (lecithinase/phospholipase C) causing myonecrosis with gas production in deep traumatic wounds. The infection progresses rapidly with crepitus, systemic toxicity, and high mortality if untreated.

Question 33

Which organism shows "tumbling motility" at 25°C but not at 37°C?

Accepted Answer

Listeria monocytogenes — Listeria exhibits characteristic tumbling or end-over-end motility at room temperature (20-25°C) due to peritrichous flagella expression. Motility is reduced or absent at 37°C, which helps distinguish it from other motile gram-positive rods.

Question 34

"Satellite phenomenon" on blood agar indicates

Accepted Answer

Staphylococcus aureus providing factors for Haemophilus — Haemophilus influenzae grows as small satellite colonies around S. aureus colonies on blood agar. S. aureus provides V factor (NAD) through hemolysis and heating of blood, while Haemophilus requires both X (hemin) and V factors.

Question 35

A patient with facial nerve palsy, meningitis, and rash has CSF showing gram-negative diplococci. The organism is

Accepted Answer

Neisseria meningitidis — N. meningitidis causes meningococcemia with petechial/purpuric rash, meningitis, and can affect cranial nerves including the facial nerve. The organism appears as gram-negative kidney bean-shaped diplococci in CSF.

Question 36

"Double zone hemolysis" on blood agar is characteristic of

Accepted Answer

Clostridium perfringens — C. perfringens shows double zone hemolysis: an inner zone of complete beta-hemolysis (due to theta toxin) and an outer zone of incomplete hemolysis (due to alpha toxin/lecithinase). This pattern is diagnostic.

Question 37

A diabetic patient develops malignant external otitis. The organism is

Accepted Answer

Pseudomonas aeruginosa — P. aeruginosa causes severe, invasive external otitis in diabetic and elderly immunocompromised patients. The infection extends from ear canal to temporal bone (osteomyelitis) and can involve cranial nerves with high morbidity.

Question 38

Which organism is both oxidase and catalase positive?

Accepted Answer

Pseudomonas aeruginosa — P. aeruginosa is both oxidase positive (produces cytochrome c oxidase) and catalase positive (produces catalase enzyme). Most Enterobacteriaceae like E. coli are oxidase negative but catalase positive.

Question 39

"Leonine facies" (lion-like face) is associated with

Accepted Answer

Lepromatous leprosy — Lepromatous leprosy causes progressive facial skin thickening, nodule formation, and loss of eyebrows creating a characteristic lion-like appearance (leonine facies). This is caused by Mycobacterium leprae, though the question relates to bacterial morphology concepts.

Question 40

A food handler with boils transmits food poisoning with rapid onset vomiting. The organism is

Accepted Answer

Staphylococcus aureus — S. aureus produces heat-stable enterotoxin in contaminated food causing rapid onset (2-6 hours) food poisoning characterized by severe nausea, vomiting, and abdominal cramps. The toxin is preformed and not destroyed by reheating.

Question 41

"Kanagawa phenomenon" (beta-hemolysis on Wagatsuma agar) is positive for pathogenic strains of

Accepted Answer

Vibrio parahaemolyticus — Kanagawa test differentiates pathogenic (hemolytic, toxin-producing) from non-pathogenic V. parahaemolyticus strains. Pathogenic strains produce thermostable direct hemolysin (TDH) causing beta-hemolysis on special Wagatsuma blood agar.

Question 42

Which organism ferments lactose slowly (late lactose fermenter)?

Accepted Answer

Citrobacter freundii — Citrobacter is a late lactose fermenter, taking more than 24 hours to ferment lactose on MacConkey agar. It initially appears as colorless (non-lactose fermenting) colonies that gradually turn pink, unlike E. coli which ferments lactose rapidly.

Question 43

A patient with chronic gastritis and peptic ulcer has a urease-positive curved rod. The organism is

Accepted Answer

Helicobacter pylori — H. pylori produces abundant urease enzyme creating an alkaline microenvironment that neutralizes gastric acid, allowing colonization of gastric mucosa. It causes chronic gastritis, peptic ulcers, and is associated with gastric cancer.

Question 44

"Strawberry tongue" and "sandpaper rash" are features of infection with

Accepted Answer

Streptococcus pyogenes — Scarlet fever, caused by erythrogenic toxin-producing Group A Streptococcus (S. pyogenes), presents with strawberry tongue (red with prominent papillae), circumoral pallor, and fine sandpaper-like rash that blanches on pressure.

Question 45

Which organism shows "school of fish" arrangement in tissue sections?

Accepted Answer

Fusobacterium necrophorum — Fusobacterium, a fusiform anaerobic gram-negative bacillus, shows characteristic school of fish arrangement with spindle-shaped cells having tapered ends aligned parallel in one direction. It causes Lemierre's syndrome and necrotizing infections.

Question 46

A patient develops bloody diarrhea after antibiotic use for H. pylori. Stool examination shows pseudomembranes. The diagnosis is confirmed by detecting

Accepted Answer

Cytotoxin assay for toxin B — C. difficile infection is confirmed by detecting cytotoxin B (more specific than toxin A) using cell culture cytotoxicity assay, enzyme immunoassay for toxins A/B, or molecular tests detecting toxin genes. Toxin B is essential for virulence.

Question 47

Which organism produces "draughtboard" or "pavement stone" colonies?

Accepted Answer

Neisseria gonorrhoeae — N. gonorrhoeae produces distinctive colony morphology types (T1-T5) on chocolate agar, with virulent forms showing small, raised colonies creating a draughtboard or pavement stone pattern. Colony type correlates with pilus expression and virulence.

Question 48

A neonate born to a mother with prolonged rupture of membranes develops sepsis. The organism showing beta-hemolysis and being CAMP positive is

Accepted Answer

Streptococcus agalactiae — Group B Streptococcus (S. agalactiae) causes early-onset neonatal sepsis and meningitis, particularly after prolonged membrane rupture. It is identified by beta-hemolysis, CAMP test positivity, and resistance to bacitracin.

Question 49

"Sulfur granules" in pus are characteristic of

Accepted Answer

Actinomyces israelii — Actinomyces forms yellow "sulfur granules" (actually microcolonies with radiating filaments, not containing actual sulfur) in draining sinuses of cervicofacial actinomycosis. These granules contain gram-positive branching filaments with peripheral clubs.

Question 50

A patient with cystic fibrosis has recurrent pulmonary infections with mucoid colonies. The organism is

Accepted Answer

Pseudomonas aeruginosa — P. aeruginosa forms mucoid biofilm due to overproduction of alginate polysaccharide in CF patients' lungs, causing chronic infection. The mucoid phenotype is characteristic of CF-associated P. aeruginosa and indicates poor prognosis with difficult eradication. END OF MCQs

MEDICAL BACTERIOLOGY (MBMM3311) - MCQs | MCQ Quiz | OmpathStudy Kenya

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