Keiser University
Course Number: MCB2000C
Course Title: Microbiology I
Credit Hours:4.0
Exam: Midterm
Date:2026
What is the MOST likely causative organism?
A. Streptococcus pyogenes B. Methicillin-resistant Staphylococcus aureus (MRSA) C.
Enterococcus faecalis D. Staphylococcus epidermidis
Correct Answer: B Rationale: Gram-positive cocci in clusters that are coagulase-positive identify
the organism as Staphylococcus aureus. Methicillin resistance further classifies it as MRSA. S.
epidermidis is coagulase-negative. Streptococci appear in chains, not clusters. MRSA is a major
pathogen in diabetic foot infections due to impaired immunity and circulation in diabetic patients.
A 3-year-old child presents with a 3-day history of bloody diarrhea, abdominal cramps, and low-
grade fever. The mother reports the child recently ate undercooked ground beef at a family
barbecue. Laboratory stool culture reveals a non-lactose-fermenting, gram-negative rod that
produces Shiga toxin.
Which of the following pathogens is MOST consistent with this clinical presentation?
A. Salmonella typhi B. Vibrio cholerae C. Escherichia coli O157:H7 D. Clostridium difficile
Correct Answer: C Rationale: E. coli O157:H7 is an enterohemorrhagic strain that produces
Shiga toxin, causing bloody diarrhea, and is strongly associated with undercooked ground beef. It
is a non-lactose fermenter on MacConkey agar. V. cholerae causes profuse watery (rice-water)
diarrhea without blood. Salmonella typhi causes enteric fever, not primarily bloody diarrhea. C.
difficile is associated with antibiotic use, not food sources.
A 28-year-old nursing student sustains a needlestick injury while drawing blood from a patient
known to be hepatitis B surface antigen (HBsAg) positive. The student has never been vaccinated
against Hepatitis B.
1
,What is the MOST appropriate immediate post-exposure management?
A. Administer hepatitis B vaccine alone within 24 hours B. Administer hepatitis B immunoglobulin
(HBIG) alone C. Administer both HBIG and begin the hepatitis B vaccine series within 24 hours
D. Wait for the student's HBsAg results before initiating any treatment
Correct Answer: C Rationale: For an unvaccinated individual exposed to HBsAg-positive blood,
both HBIG (for immediate passive immunity) and hepatitis B vaccine (for long-term active
immunity) should be given as soon as possible, ideally within 24 hours. HBIG alone provides only
temporary protection. Delaying treatment to await test results significantly increases the risk of
infection.
A 60-year-old immunocompromised patient on long-term corticosteroid therapy develops a
pulmonary infection. Bronchoalveolar lavage reveals a fungus with narrow-based budding yeast
cells encapsulated in a thick polysaccharide capsule. India ink preparation confirms the finding.
Which organism is responsible for this infection?
A. Aspergillus fumigatus B. Candida albicans C. Cryptococcus neoformans D. Histoplasma
capsulatum
Correct Answer: C Rationale: Cryptococcus neoformans is characterized by its thick
polysaccharide capsule, which is highlighted by India ink preparation (the capsule appears as a
clear halo around the yeast cell). It causes life-threatening pneumonia and meningitis in
immunocompromised individuals. Aspergillus forms hyphae and fruiting bodies. Candida produces
pseudohyphae and germ tubes. Histoplasma is an intracellular yeast found within macrophages.
A 22-year-old sexually active female presents with dysuria, purulent cervical discharge, and lower
abdominal pain. Gram stain of the discharge shows gram-negative diplococci within neutrophils.
She reports a penicillin allergy.
What is the MOST appropriate treatment for this patient?
A. Oral amoxicillin-clavulanate B. Intramuscular ceftriaxone plus oral azithromycin C. Oral
metronidazole D. Intravenous vancomycin
Correct Answer: B Rationale: The presentation is consistent with gonorrhea caused by Neisseria
gonorrhoeae (gram-negative intracellular diplococci). Current CDC guidelines recommend dual
therapy with ceftriaxone IM and azithromycin orally (also to cover concurrent Chlamydia).
Cephalosporins (like ceftriaxone) can generally be used cautiously even in penicillin-allergic
patients unless there is a severe allergy. Metronidazole covers anaerobes/protozoa. Vancomycin is
for gram-positive organisms.
2
,A 70-year-old male patient who has been hospitalized for 10 days and treated with broad-spectrum
antibiotics develops watery diarrhea, fever, and abdominal cramping. Stool PCR tests positive for
toxin-producing bacteria. Colonoscopy reveals pseudomembranous colitis.
What virulence factors produced by this organism are DIRECTLY responsible for the colonic
damage?
A. Staphylococcal enterotoxin A and B B. Toxin A (enterotoxin) and Toxin B (cytotoxin) C.
Verotoxin and hemolysin D. Exfoliative toxin and toxic shock syndrome toxin
Correct Answer: B Rationale: Clostridium difficile produces two main toxins: Toxin A (an
enterotoxin causing fluid secretion and mucosal damage) and Toxin B (a more potent cytotoxin
causing direct cell death and pseudomembrane formation). The disruption of normal flora by
broad-spectrum antibiotics allows C. difficile to proliferate. The other toxins listed are associated
with S. aureus and E. coli, not C. difficile.
A neonate born to a mother with a history of untreated genital herpes develops vesicular lesions,
fever, and seizures at 7 days of life. CSF analysis shows lymphocytic pleocytosis and elevated
protein. PCR of the CSF is positive for Herpes Simplex Virus type 2 (HSV-2).
Which of the following statements about HSV pathogenesis is MOST accurate?
A. HSV-2 establishes latency in the trigeminal ganglia B. Neonatal herpes is most commonly
acquired postnatally through respiratory droplets C. HSV-2 establishes latency in the sacral dorsal
root ganglia and reactivates during stress or immunosuppression D. HSV-2 is a DNA virus that
replicates exclusively in the cytoplasm
Correct Answer: C Rationale: HSV-2 establishes lifelong latency in the sacral dorsal root ganglia
after primary genital infection. Reactivation occurs due to stress, fever, immunosuppression, or
hormonal changes. Neonatal herpes is primarily acquired during passage through an infected birth
canal (perinatal transmission). HSV-1 (not HSV-2) typically establishes latency in the trigeminal
ganglia. HSV replicates in the nucleus, not the cytoplasm.
A 55-year-old male farmer from western Kenya presents with fever, chills, headache, and jaundice.
Blood smear reveals ring-form trophozoites with "appliqué" (accolé) appearance and multiple
infection of red blood cells. The parasites are small relative to the size of the host RBCs.
Which Plasmodium species is MOST likely responsible, and why is this case particularly
dangerous?
A. Plasmodium vivax; because it causes relapsing malaria B. Plasmodium falciparum; because it
causes cytoadherence and cerebral malaria C. Plasmodium malariae; because it causes nephrotic
syndrome D. Plasmodium ovale; because it has a long incubation period
Correct Answer: B Rationale: P. falciparum is identified by its ring-form trophozoites with the
appliqué appearance, multiple ring forms per RBC, and no enlargement of host RBCs. It is the
3
, most deadly species because it produces a protein (PfEMP1) that causes infected RBCs to adhere to
vascular endothelium (cytoadherence), leading to microvascular occlusion and potentially fatal
cerebral malaria. P. vivax and P. ovale form hypnozoites causing relapse.
A 35-year-old HIV-positive patient with a CD4 count of 50 cells/μL presents with painless white
lesions on the lateral borders of the tongue that cannot be scraped off. Biopsy reveals epithelial
proliferation with viral inclusions.
What is the MOST likely diagnosis and the causative virus?
A. Oral candidiasis caused by Candida albicans B. Oral hairy leukoplakia caused by Epstein-Barr
Virus (EBV) C. Herpangina caused by Coxsackievirus A D. Aphthous ulcers caused by HSV-1
Correct Answer: B Rationale: Oral hairy leukoplakia is caused by EBV reactivation in severely
immunocompromised individuals (CD4 < 200). The lesions are white, corrugated (hairy), and
located on the lateral tongue — they CANNOT be scraped off, distinguishing them from oral
candidiasis (which can be scraped). Candida lesions leave a bleeding base when scraped.
Herpangina presents with painful vesicles on the soft palate, not lateral tongue.
A 40-year-old intravenous drug user presents with fever, new-onset heart murmur, and splinter
hemorrhages under the fingernails. Blood cultures drawn from two separate sites grow gram-
positive cocci in clusters. Echocardiography confirms vegetations on the tricuspid valve.
Which statement about the pathogenesis of this condition is MOST accurate?
A. The infection most commonly affects the mitral valve in IV drug users B. The organism
produces protein A, which binds IgG and evades complement-mediated killing C. The vegetations
are sterile and consist only of fibrin and platelets D. This condition is caused exclusively by
viridans streptococci
Correct Answer: B Rationale: This is infective endocarditis caused by S. aureus (gram-positive
cocci in clusters). S. aureus produces Protein A, which binds the Fc region of IgG, blocking
opsonization and complement activation — a key immune evasion mechanism. In IV drug users,
right-sided (tricuspid valve) endocarditis is more common due to direct inoculation into the venous
system. Vegetations are NOT sterile; they contain bacteria embedded in fibrin-platelet matrices.
A 25-year-old female presents with fever, stiff neck, photophobia, and a non-blanching petechial
rash that is rapidly spreading. CSF shows: WBC 15,000/μL (predominantly neutrophils), protein
200 mg/dL, glucose 20 mg/dL (serum glucose 90 mg/dL). Gram stain shows gram-negative
diplococci.
What is the MOST immediate priority in managing this patient?
4
Course Number: MCB2000C
Course Title: Microbiology I
Credit Hours:4.0
Exam: Midterm
Date:2026
What is the MOST likely causative organism?
A. Streptococcus pyogenes B. Methicillin-resistant Staphylococcus aureus (MRSA) C.
Enterococcus faecalis D. Staphylococcus epidermidis
Correct Answer: B Rationale: Gram-positive cocci in clusters that are coagulase-positive identify
the organism as Staphylococcus aureus. Methicillin resistance further classifies it as MRSA. S.
epidermidis is coagulase-negative. Streptococci appear in chains, not clusters. MRSA is a major
pathogen in diabetic foot infections due to impaired immunity and circulation in diabetic patients.
A 3-year-old child presents with a 3-day history of bloody diarrhea, abdominal cramps, and low-
grade fever. The mother reports the child recently ate undercooked ground beef at a family
barbecue. Laboratory stool culture reveals a non-lactose-fermenting, gram-negative rod that
produces Shiga toxin.
Which of the following pathogens is MOST consistent with this clinical presentation?
A. Salmonella typhi B. Vibrio cholerae C. Escherichia coli O157:H7 D. Clostridium difficile
Correct Answer: C Rationale: E. coli O157:H7 is an enterohemorrhagic strain that produces
Shiga toxin, causing bloody diarrhea, and is strongly associated with undercooked ground beef. It
is a non-lactose fermenter on MacConkey agar. V. cholerae causes profuse watery (rice-water)
diarrhea without blood. Salmonella typhi causes enteric fever, not primarily bloody diarrhea. C.
difficile is associated with antibiotic use, not food sources.
A 28-year-old nursing student sustains a needlestick injury while drawing blood from a patient
known to be hepatitis B surface antigen (HBsAg) positive. The student has never been vaccinated
against Hepatitis B.
1
,What is the MOST appropriate immediate post-exposure management?
A. Administer hepatitis B vaccine alone within 24 hours B. Administer hepatitis B immunoglobulin
(HBIG) alone C. Administer both HBIG and begin the hepatitis B vaccine series within 24 hours
D. Wait for the student's HBsAg results before initiating any treatment
Correct Answer: C Rationale: For an unvaccinated individual exposed to HBsAg-positive blood,
both HBIG (for immediate passive immunity) and hepatitis B vaccine (for long-term active
immunity) should be given as soon as possible, ideally within 24 hours. HBIG alone provides only
temporary protection. Delaying treatment to await test results significantly increases the risk of
infection.
A 60-year-old immunocompromised patient on long-term corticosteroid therapy develops a
pulmonary infection. Bronchoalveolar lavage reveals a fungus with narrow-based budding yeast
cells encapsulated in a thick polysaccharide capsule. India ink preparation confirms the finding.
Which organism is responsible for this infection?
A. Aspergillus fumigatus B. Candida albicans C. Cryptococcus neoformans D. Histoplasma
capsulatum
Correct Answer: C Rationale: Cryptococcus neoformans is characterized by its thick
polysaccharide capsule, which is highlighted by India ink preparation (the capsule appears as a
clear halo around the yeast cell). It causes life-threatening pneumonia and meningitis in
immunocompromised individuals. Aspergillus forms hyphae and fruiting bodies. Candida produces
pseudohyphae and germ tubes. Histoplasma is an intracellular yeast found within macrophages.
A 22-year-old sexually active female presents with dysuria, purulent cervical discharge, and lower
abdominal pain. Gram stain of the discharge shows gram-negative diplococci within neutrophils.
She reports a penicillin allergy.
What is the MOST appropriate treatment for this patient?
A. Oral amoxicillin-clavulanate B. Intramuscular ceftriaxone plus oral azithromycin C. Oral
metronidazole D. Intravenous vancomycin
Correct Answer: B Rationale: The presentation is consistent with gonorrhea caused by Neisseria
gonorrhoeae (gram-negative intracellular diplococci). Current CDC guidelines recommend dual
therapy with ceftriaxone IM and azithromycin orally (also to cover concurrent Chlamydia).
Cephalosporins (like ceftriaxone) can generally be used cautiously even in penicillin-allergic
patients unless there is a severe allergy. Metronidazole covers anaerobes/protozoa. Vancomycin is
for gram-positive organisms.
2
,A 70-year-old male patient who has been hospitalized for 10 days and treated with broad-spectrum
antibiotics develops watery diarrhea, fever, and abdominal cramping. Stool PCR tests positive for
toxin-producing bacteria. Colonoscopy reveals pseudomembranous colitis.
What virulence factors produced by this organism are DIRECTLY responsible for the colonic
damage?
A. Staphylococcal enterotoxin A and B B. Toxin A (enterotoxin) and Toxin B (cytotoxin) C.
Verotoxin and hemolysin D. Exfoliative toxin and toxic shock syndrome toxin
Correct Answer: B Rationale: Clostridium difficile produces two main toxins: Toxin A (an
enterotoxin causing fluid secretion and mucosal damage) and Toxin B (a more potent cytotoxin
causing direct cell death and pseudomembrane formation). The disruption of normal flora by
broad-spectrum antibiotics allows C. difficile to proliferate. The other toxins listed are associated
with S. aureus and E. coli, not C. difficile.
A neonate born to a mother with a history of untreated genital herpes develops vesicular lesions,
fever, and seizures at 7 days of life. CSF analysis shows lymphocytic pleocytosis and elevated
protein. PCR of the CSF is positive for Herpes Simplex Virus type 2 (HSV-2).
Which of the following statements about HSV pathogenesis is MOST accurate?
A. HSV-2 establishes latency in the trigeminal ganglia B. Neonatal herpes is most commonly
acquired postnatally through respiratory droplets C. HSV-2 establishes latency in the sacral dorsal
root ganglia and reactivates during stress or immunosuppression D. HSV-2 is a DNA virus that
replicates exclusively in the cytoplasm
Correct Answer: C Rationale: HSV-2 establishes lifelong latency in the sacral dorsal root ganglia
after primary genital infection. Reactivation occurs due to stress, fever, immunosuppression, or
hormonal changes. Neonatal herpes is primarily acquired during passage through an infected birth
canal (perinatal transmission). HSV-1 (not HSV-2) typically establishes latency in the trigeminal
ganglia. HSV replicates in the nucleus, not the cytoplasm.
A 55-year-old male farmer from western Kenya presents with fever, chills, headache, and jaundice.
Blood smear reveals ring-form trophozoites with "appliqué" (accolé) appearance and multiple
infection of red blood cells. The parasites are small relative to the size of the host RBCs.
Which Plasmodium species is MOST likely responsible, and why is this case particularly
dangerous?
A. Plasmodium vivax; because it causes relapsing malaria B. Plasmodium falciparum; because it
causes cytoadherence and cerebral malaria C. Plasmodium malariae; because it causes nephrotic
syndrome D. Plasmodium ovale; because it has a long incubation period
Correct Answer: B Rationale: P. falciparum is identified by its ring-form trophozoites with the
appliqué appearance, multiple ring forms per RBC, and no enlargement of host RBCs. It is the
3
, most deadly species because it produces a protein (PfEMP1) that causes infected RBCs to adhere to
vascular endothelium (cytoadherence), leading to microvascular occlusion and potentially fatal
cerebral malaria. P. vivax and P. ovale form hypnozoites causing relapse.
A 35-year-old HIV-positive patient with a CD4 count of 50 cells/μL presents with painless white
lesions on the lateral borders of the tongue that cannot be scraped off. Biopsy reveals epithelial
proliferation with viral inclusions.
What is the MOST likely diagnosis and the causative virus?
A. Oral candidiasis caused by Candida albicans B. Oral hairy leukoplakia caused by Epstein-Barr
Virus (EBV) C. Herpangina caused by Coxsackievirus A D. Aphthous ulcers caused by HSV-1
Correct Answer: B Rationale: Oral hairy leukoplakia is caused by EBV reactivation in severely
immunocompromised individuals (CD4 < 200). The lesions are white, corrugated (hairy), and
located on the lateral tongue — they CANNOT be scraped off, distinguishing them from oral
candidiasis (which can be scraped). Candida lesions leave a bleeding base when scraped.
Herpangina presents with painful vesicles on the soft palate, not lateral tongue.
A 40-year-old intravenous drug user presents with fever, new-onset heart murmur, and splinter
hemorrhages under the fingernails. Blood cultures drawn from two separate sites grow gram-
positive cocci in clusters. Echocardiography confirms vegetations on the tricuspid valve.
Which statement about the pathogenesis of this condition is MOST accurate?
A. The infection most commonly affects the mitral valve in IV drug users B. The organism
produces protein A, which binds IgG and evades complement-mediated killing C. The vegetations
are sterile and consist only of fibrin and platelets D. This condition is caused exclusively by
viridans streptococci
Correct Answer: B Rationale: This is infective endocarditis caused by S. aureus (gram-positive
cocci in clusters). S. aureus produces Protein A, which binds the Fc region of IgG, blocking
opsonization and complement activation — a key immune evasion mechanism. In IV drug users,
right-sided (tricuspid valve) endocarditis is more common due to direct inoculation into the venous
system. Vegetations are NOT sterile; they contain bacteria embedded in fibrin-platelet matrices.
A 25-year-old female presents with fever, stiff neck, photophobia, and a non-blanching petechial
rash that is rapidly spreading. CSF shows: WBC 15,000/μL (predominantly neutrophils), protein
200 mg/dL, glucose 20 mg/dL (serum glucose 90 mg/dL). Gram stain shows gram-negative
diplococci.
What is the MOST immediate priority in managing this patient?
4
