SCL-251 Microbiology
Examination Practice Questions &
Verified Rationales
Section 1: Clinical Bacteriology & Diagnostic Methods
Q1. A 22-year-old college student presents with a sore throat, fever, and swollen neck
glands. Physical examination shows tonsillar exudates, and a rapid strep test comes
back positive. What microscopic appearance and laboratory test results match the
bacteria causing this condition?
A) Gram-positive cocci in chains, beta-hemolytic, sensitive to bacitracin
B) Gram-positive cocci in clusters, catalase positive, coagulase positive
C) Gram-negative diplococci, oxidase positive, ferments maltose
D) Gram-positive rods, spore-forming, strict anaerobe
Correct Answer: A) Gram-positive cocci in chains, beta-hemolytic, sensitive to bacitracin
Rationale: Streptococcus pyogenes (Group A Strep) causes strep throat and grows in chains,
completely clears blood cells on an agar plate (beta-hemolysis), and stops growing near a
bacitracin disk. Staphylococcus aureus also causes throat and skin infections, but it grows in
clusters like grapes and tests catalase-positive.
Q2. A 68-year-old hospital patient develops severe watery diarrhea and abdominal
cramps after completing a 2-week course of clindamycin. Which organism is most likely
responsible for this intestinal infection?
A) Escherichia coli O157:H7
B) Clostridioides difficile
C) Shigella sonnei
D) Campylobacter jejuni
Correct Answer: B) Clostridioides difficile
Rationale: Broad-spectrum antibiotics like clindamycin wipe out healthy gut flora, allowing
Clostridioides difficile to overgrow and release toxins that inflame the colon. E. coli O157:H7
causes bloody diarrhea instead and usually comes from contaminated food rather than antibiotic
use.
Q3. A landscaper notices an expanding red patch on his leg that looks like a target or
bullseye two weeks after pulling a tick off his skin. What bacterium causes this
characteristic skin rash?
A) Rickettsia rickettsii
B) Francisella tularensis
C) Borrelia burgdorferi
D) Coxiella burnetii
Correct Answer: C) Borrelia burgdorferi
Rationale: Borrelia burgdorferi is a spiral-shaped bacterium transmitted by blacklegged ticks
that causes Lyme disease, which typically starts with the bullseye rash known as erythema
migrans. Rickettsia rickettsii causes Rocky Mountain spotted fever, which produces a spotty
, rash on the wrists and ankles instead.
Q4. A newborn develops a high fever, extreme sleepiness, and poor feeding one day after
birth. Samples of spinal fluid show Gram-positive rods that show a tumbling motion
when viewed under a microscope at room temperature. How did the mother most likely
catch this organism?
A) Handling dried bird droppings in the yard
B) Cleaning out a domestic cat's litter box
C) Getting bitten by an infected mosquito
D) Eating unpasteurized soft cheese or deli meats
Correct Answer: D) Eating unpasteurized soft cheese or deli meats
Rationale: Listeria monocytogenes can survive cold temperatures in contaminated
unpasteurized dairy products and processed cold cuts, passing from an infected mother to her
baby during pregnancy or delivery. Cat litter boxes carry Toxoplasma gondii parasites, not
tumbling bacterial rods.
Q5. A young adult visits a clinic complaining of a burning sensation during urination and
frequent trips to the bathroom. A urine culture grows Gram-negative rods that ferment
lactose and test positive for indole production. What bacterial structure helps this bug
stick to the lining of the urinary tract?
A) P-fimbriae (pili)
B) Flagella
C) Thick peptidoglycan wall
D) Endospores
Correct Answer: A) P-fimbriae (pili)
Rationale: Uropathogenic Escherichia coli uses tiny hair-like projections called P-fimbriae to
lock onto bladder wall cells so urine flow doesn't flush them out. Flagella help bacteria swim
around rather than anchor down tightly to mucosal surfaces.
Q6. A patient in the ICU develops a surgical wound infection. Culture results show Gram-
negative rods that produce a blue-green pigment and smell distinctly like sweet grapes.
What is the main toxin this bacterium uses to damage host tissues?
A) Exotoxin A
B) Alpha toxin
C) Shiga toxin
D) Toxic shock syndrome toxin-1
Correct Answer: A) Exotoxin A
Rationale: Pseudomonas aeruginosa produces colorful pyocyanin pigments, a fruity aroma,
and Exotoxin A, which stops host cells from making essential proteins. Clostridium perfringens
makes alpha toxin, while Staphylococcus aureus makes toxic shock syndrome toxin-1.
Q7. A college student gets rushed to the hospital with a sudden high fever, stiff neck,
severe headache, and purple spots on his skin. Microscopic testing of spinal fluid
reveals Gram-negative diplococci. What special agar plate is used to isolate this bug
from close contacts?
A) Sabouraud dextrose agar
B) Thayer-Martin selective agar
C) MacConkey agar
D) Lowenstein-Jensen medium
Correct Answer: B) Thayer-Martin selective agar
Rationale: Neisseria meningitidis is a delicate Gram-negative diplococcus that requires Thayer-
Martin agar (chocolate agar mixed with antibiotics) to keep normal throat bacteria from
swamping the culture. MacConkey agar isolates intestinal Gram-negative rods like E. coli, while
Sabouraud agar grows fungi.
Q8. An elderly man with a long history of smoking develops a high fever, dry cough,
Examination Practice Questions &
Verified Rationales
Section 1: Clinical Bacteriology & Diagnostic Methods
Q1. A 22-year-old college student presents with a sore throat, fever, and swollen neck
glands. Physical examination shows tonsillar exudates, and a rapid strep test comes
back positive. What microscopic appearance and laboratory test results match the
bacteria causing this condition?
A) Gram-positive cocci in chains, beta-hemolytic, sensitive to bacitracin
B) Gram-positive cocci in clusters, catalase positive, coagulase positive
C) Gram-negative diplococci, oxidase positive, ferments maltose
D) Gram-positive rods, spore-forming, strict anaerobe
Correct Answer: A) Gram-positive cocci in chains, beta-hemolytic, sensitive to bacitracin
Rationale: Streptococcus pyogenes (Group A Strep) causes strep throat and grows in chains,
completely clears blood cells on an agar plate (beta-hemolysis), and stops growing near a
bacitracin disk. Staphylococcus aureus also causes throat and skin infections, but it grows in
clusters like grapes and tests catalase-positive.
Q2. A 68-year-old hospital patient develops severe watery diarrhea and abdominal
cramps after completing a 2-week course of clindamycin. Which organism is most likely
responsible for this intestinal infection?
A) Escherichia coli O157:H7
B) Clostridioides difficile
C) Shigella sonnei
D) Campylobacter jejuni
Correct Answer: B) Clostridioides difficile
Rationale: Broad-spectrum antibiotics like clindamycin wipe out healthy gut flora, allowing
Clostridioides difficile to overgrow and release toxins that inflame the colon. E. coli O157:H7
causes bloody diarrhea instead and usually comes from contaminated food rather than antibiotic
use.
Q3. A landscaper notices an expanding red patch on his leg that looks like a target or
bullseye two weeks after pulling a tick off his skin. What bacterium causes this
characteristic skin rash?
A) Rickettsia rickettsii
B) Francisella tularensis
C) Borrelia burgdorferi
D) Coxiella burnetii
Correct Answer: C) Borrelia burgdorferi
Rationale: Borrelia burgdorferi is a spiral-shaped bacterium transmitted by blacklegged ticks
that causes Lyme disease, which typically starts with the bullseye rash known as erythema
migrans. Rickettsia rickettsii causes Rocky Mountain spotted fever, which produces a spotty
, rash on the wrists and ankles instead.
Q4. A newborn develops a high fever, extreme sleepiness, and poor feeding one day after
birth. Samples of spinal fluid show Gram-positive rods that show a tumbling motion
when viewed under a microscope at room temperature. How did the mother most likely
catch this organism?
A) Handling dried bird droppings in the yard
B) Cleaning out a domestic cat's litter box
C) Getting bitten by an infected mosquito
D) Eating unpasteurized soft cheese or deli meats
Correct Answer: D) Eating unpasteurized soft cheese or deli meats
Rationale: Listeria monocytogenes can survive cold temperatures in contaminated
unpasteurized dairy products and processed cold cuts, passing from an infected mother to her
baby during pregnancy or delivery. Cat litter boxes carry Toxoplasma gondii parasites, not
tumbling bacterial rods.
Q5. A young adult visits a clinic complaining of a burning sensation during urination and
frequent trips to the bathroom. A urine culture grows Gram-negative rods that ferment
lactose and test positive for indole production. What bacterial structure helps this bug
stick to the lining of the urinary tract?
A) P-fimbriae (pili)
B) Flagella
C) Thick peptidoglycan wall
D) Endospores
Correct Answer: A) P-fimbriae (pili)
Rationale: Uropathogenic Escherichia coli uses tiny hair-like projections called P-fimbriae to
lock onto bladder wall cells so urine flow doesn't flush them out. Flagella help bacteria swim
around rather than anchor down tightly to mucosal surfaces.
Q6. A patient in the ICU develops a surgical wound infection. Culture results show Gram-
negative rods that produce a blue-green pigment and smell distinctly like sweet grapes.
What is the main toxin this bacterium uses to damage host tissues?
A) Exotoxin A
B) Alpha toxin
C) Shiga toxin
D) Toxic shock syndrome toxin-1
Correct Answer: A) Exotoxin A
Rationale: Pseudomonas aeruginosa produces colorful pyocyanin pigments, a fruity aroma,
and Exotoxin A, which stops host cells from making essential proteins. Clostridium perfringens
makes alpha toxin, while Staphylococcus aureus makes toxic shock syndrome toxin-1.
Q7. A college student gets rushed to the hospital with a sudden high fever, stiff neck,
severe headache, and purple spots on his skin. Microscopic testing of spinal fluid
reveals Gram-negative diplococci. What special agar plate is used to isolate this bug
from close contacts?
A) Sabouraud dextrose agar
B) Thayer-Martin selective agar
C) MacConkey agar
D) Lowenstein-Jensen medium
Correct Answer: B) Thayer-Martin selective agar
Rationale: Neisseria meningitidis is a delicate Gram-negative diplococcus that requires Thayer-
Martin agar (chocolate agar mixed with antibiotics) to keep normal throat bacteria from
swamping the culture. MacConkey agar isolates intestinal Gram-negative rods like E. coli, while
Sabouraud agar grows fungi.
Q8. An elderly man with a long history of smoking develops a high fever, dry cough,
