BIO 250 Exam 4 V3 | BIO 250 Microbiology | Actual Q&A with
Rationale (BIO250 Exam 4) | StraighterLine
1. Which of the following mechanisms best describes the action of penicillin on a bacterial
cell?
A. Blockage of the cross-linking of peptidoglycan layers in the cell wall.
B. Disruption of the cytoplasmic membrane integrity.
C. Inhibition of protein synthesis by binding to the 30S ribosomal subunit.
D. Inhibition of DNA gyrase activity.
Correct Answer: A
Explanation: Penicillin acts by binding to penicillin-binding proteins (PBPs) which are
enzymes involved in the synthesis of peptidoglycan. By inhibiting these enzymes, the drug
prevents the cross-linking of the glycan chains, leading to a weakened cell wall. The
resulting osmotic pressure causes the bacterial cell to undergo lysis and death.
2. Which of the following cell types are considered professional phagocytes involved in the
innate immune response? (Select all that apply)
A. Options A, B, and D only.
B. Macrophages
C. B-lymphocytes
D. Dendritic cells
E. Natural Killer (NK) cells
F. Neutrophils
Correct Answer: A
Explanation: Professional phagocytes are specialized cells that ingest and kill invading
pathogens efficiently. Neutrophils are the first responders, while macrophages and
dendritic cells also play crucial roles in phagocytosis and antigen presentation. B-
lymphocytes and NK cells have different primary functions, such as antibody production
and direct lysis of infected cells, respectively.
3. A patient presents with a fever, malaise, and a sore throat. This stage of an infectious
disease, characterized by general, non-specific symptoms, is known as the:
A. Incubation period
B. Prodromal stage
,C. Period of invasion
D. Convalescent period
Correct Answer: B
Explanation: The prodromal stage is the short period following the incubation period
characterized by the onset of vague, generalized symptoms. During this time, the pathogen
is multiplying but has not yet reached high enough levels to cause specific signs of disease.
It often represents the earliest notification that the host is becoming ill.
4. The concept of ‘selective toxicity’ in antimicrobial therapy refers to the ability of a drug to:
A. Kill all microbes regardless of the host species.
B. Kill or inhibit microbial cells without damaging host tissues.
C. Act as a broad-spectrum agent against both Gram-positive and Gram-negative bacteria.
D. Remain active in the presence of organic matter.
Correct Answer: B
Explanation: Selective toxicity is a central principle of chemotherapy where a drug targets
a specific structure or metabolic pathway present in the microbe but absent in the human
host. This ensures that the pathogen is eliminated while side effects for the patient are
minimized. A high therapeutic index is generally associated with drugs that possess high
selective toxicity.
5. Which of the following are recognized classic signs and symptoms of inflammation?
A. Redness, warmth, swelling, and pain.
B. Redness, coldness, swelling, and pain.
C. Pallor, warmth, numbness, and itching.
D. Redness, warmth, hardness, and cyanosis.
Correct Answer: A
Explanation: The four classic signs of inflammation are rubor (redness), calor (warmth),
tumor (swelling), and dolor (pain). These occur due to increased blood flow (vasodilation)
and increased capillary permeability at the site of injury or infection. Loss of function is
often considered a fifth sign in more severe cases.
6. Sulfa drugs (sulfonamides) inhibit bacterial growth by acting as competitive inhibitors for
the enzyme that converts PABA into:
A. Peptidoglycan
B. Protein
C. DNA
, D. Folic acid
Correct Answer: D
Explanation: Sulfa drugs are structural analogs of para-aminobenzoic acid (PABA), a
precursor in the synthesis of dihydrofolic acid. Bacteria must synthesize their own folic
acid to produce nucleotides, whereas humans obtain it through diet. Therefore, blocking
this pathway selectively prevents bacterial DNA and RNA synthesis.
7. Which class of antibody is the most abundant in serum and is capable of crossing the
placenta to provide passive immunity to the fetus?
A. IgA
B. IgM
C. IgE
D. IgG
Correct Answer: D
Explanation: IgG is the primary antibody produced during the secondary immune
response and makes up about 80% of the total serum antibodies. It is the only
immunoglobulin class that can cross the placental barrier, providing the newborn with
natural passive immunity for the first few months of life. It also plays a key role in
opsonization and complement activation.
8. Endotoxins are characterized by which of the following features?
A. They are lipopolysaccharide (LPS) components of the Gram-negative outer membrane.
B. They are proteins secreted by living Gram-positive bacteria.
C. They have high toxicity in minute amounts and are highly specific in their action.
D. They can be converted into toxoids for use in vaccines.
Correct Answer: A
Explanation: Endotoxins, specifically the Lipid A portion of the lipopolysaccharide, are
structural components of the outer membrane of Gram-negative bacteria. Unlike exotoxins,
they are released only when the cell dies or undergoes lysis. They typically produce
systemic effects like fever and inflammation rather than targeting specific organ systems.
9. A ‘superinfection’ most likely results from:
A. The use of a broad-spectrum antibiotic that destroys the normal microbiota.
B. The use of a narrow-spectrum antibiotic that is ineffective.
C. A mutation in a viral genome making it resistant to vaccines.
D. The failure to complete a prescribed course of antibiotics.
Rationale (BIO250 Exam 4) | StraighterLine
1. Which of the following mechanisms best describes the action of penicillin on a bacterial
cell?
A. Blockage of the cross-linking of peptidoglycan layers in the cell wall.
B. Disruption of the cytoplasmic membrane integrity.
C. Inhibition of protein synthesis by binding to the 30S ribosomal subunit.
D. Inhibition of DNA gyrase activity.
Correct Answer: A
Explanation: Penicillin acts by binding to penicillin-binding proteins (PBPs) which are
enzymes involved in the synthesis of peptidoglycan. By inhibiting these enzymes, the drug
prevents the cross-linking of the glycan chains, leading to a weakened cell wall. The
resulting osmotic pressure causes the bacterial cell to undergo lysis and death.
2. Which of the following cell types are considered professional phagocytes involved in the
innate immune response? (Select all that apply)
A. Options A, B, and D only.
B. Macrophages
C. B-lymphocytes
D. Dendritic cells
E. Natural Killer (NK) cells
F. Neutrophils
Correct Answer: A
Explanation: Professional phagocytes are specialized cells that ingest and kill invading
pathogens efficiently. Neutrophils are the first responders, while macrophages and
dendritic cells also play crucial roles in phagocytosis and antigen presentation. B-
lymphocytes and NK cells have different primary functions, such as antibody production
and direct lysis of infected cells, respectively.
3. A patient presents with a fever, malaise, and a sore throat. This stage of an infectious
disease, characterized by general, non-specific symptoms, is known as the:
A. Incubation period
B. Prodromal stage
,C. Period of invasion
D. Convalescent period
Correct Answer: B
Explanation: The prodromal stage is the short period following the incubation period
characterized by the onset of vague, generalized symptoms. During this time, the pathogen
is multiplying but has not yet reached high enough levels to cause specific signs of disease.
It often represents the earliest notification that the host is becoming ill.
4. The concept of ‘selective toxicity’ in antimicrobial therapy refers to the ability of a drug to:
A. Kill all microbes regardless of the host species.
B. Kill or inhibit microbial cells without damaging host tissues.
C. Act as a broad-spectrum agent against both Gram-positive and Gram-negative bacteria.
D. Remain active in the presence of organic matter.
Correct Answer: B
Explanation: Selective toxicity is a central principle of chemotherapy where a drug targets
a specific structure or metabolic pathway present in the microbe but absent in the human
host. This ensures that the pathogen is eliminated while side effects for the patient are
minimized. A high therapeutic index is generally associated with drugs that possess high
selective toxicity.
5. Which of the following are recognized classic signs and symptoms of inflammation?
A. Redness, warmth, swelling, and pain.
B. Redness, coldness, swelling, and pain.
C. Pallor, warmth, numbness, and itching.
D. Redness, warmth, hardness, and cyanosis.
Correct Answer: A
Explanation: The four classic signs of inflammation are rubor (redness), calor (warmth),
tumor (swelling), and dolor (pain). These occur due to increased blood flow (vasodilation)
and increased capillary permeability at the site of injury or infection. Loss of function is
often considered a fifth sign in more severe cases.
6. Sulfa drugs (sulfonamides) inhibit bacterial growth by acting as competitive inhibitors for
the enzyme that converts PABA into:
A. Peptidoglycan
B. Protein
C. DNA
, D. Folic acid
Correct Answer: D
Explanation: Sulfa drugs are structural analogs of para-aminobenzoic acid (PABA), a
precursor in the synthesis of dihydrofolic acid. Bacteria must synthesize their own folic
acid to produce nucleotides, whereas humans obtain it through diet. Therefore, blocking
this pathway selectively prevents bacterial DNA and RNA synthesis.
7. Which class of antibody is the most abundant in serum and is capable of crossing the
placenta to provide passive immunity to the fetus?
A. IgA
B. IgM
C. IgE
D. IgG
Correct Answer: D
Explanation: IgG is the primary antibody produced during the secondary immune
response and makes up about 80% of the total serum antibodies. It is the only
immunoglobulin class that can cross the placental barrier, providing the newborn with
natural passive immunity for the first few months of life. It also plays a key role in
opsonization and complement activation.
8. Endotoxins are characterized by which of the following features?
A. They are lipopolysaccharide (LPS) components of the Gram-negative outer membrane.
B. They are proteins secreted by living Gram-positive bacteria.
C. They have high toxicity in minute amounts and are highly specific in their action.
D. They can be converted into toxoids for use in vaccines.
Correct Answer: A
Explanation: Endotoxins, specifically the Lipid A portion of the lipopolysaccharide, are
structural components of the outer membrane of Gram-negative bacteria. Unlike exotoxins,
they are released only when the cell dies or undergoes lysis. They typically produce
systemic effects like fever and inflammation rather than targeting specific organ systems.
9. A ‘superinfection’ most likely results from:
A. The use of a broad-spectrum antibiotic that destroys the normal microbiota.
B. The use of a narrow-spectrum antibiotic that is ineffective.
C. A mutation in a viral genome making it resistant to vaccines.
D. The failure to complete a prescribed course of antibiotics.
