Pharmacology Antimicrobial and Chemotherapeutic Agents Questions and Answers 1 — Questions and Answers
Question 1: A patient is prescribed penicillin for a gram-positive bacterial infection. What is the primary mechanism by which this antibiotic exerts its bactericidal effect?
- Inhibition of DNA gyrase, preventing bacterial DNA replication.
- Binding to the 30S ribosomal subunit to disrupt protein synthesis.
- Inhibition of transpeptidase, preventing peptidoglycan cell wall cross-linking. (Correct answer)
- Creation of pores in the bacterial cell membrane, causing leakage of intracellular contents.
Correct answer: Inhibition of transpeptidase, preventing peptidoglycan cell wall cross-linking.
Penicillins are beta-lactam antibiotics that work by irreversibly inhibiting the transpeptidase enzyme (also known as a penicillin-binding protein). This enzyme is crucial for the final step in peptidoglycan synthesis, which involves cross-linking the peptide chains to form a rigid cell wall. By blocking this process, penicillins weaken the cell wall, leading to cell lysis and bacterial death.
Question 2: A 68-year-old male with a history of atrial fibrillation, well-controlled on warfarin, is diagnosed with oropharyngeal candidiasis and is prescribed a course of oral fluconazole. What is the most significant potential interaction the pharmacist should be concerned about?
- Inhibition of warfarin metabolism, leading to a supratherapeutic INR and increased risk of bleeding. (Correct answer)
- Increased metabolism of warfarin, leading to a subtherapeutic INR and risk of thrombosis.
- Decreased absorption of fluconazole, leading to antifungal treatment failure.
- Displacement of warfarin from plasma proteins by fluconazole, causing a transient increase in free drug.
Correct answer: Inhibition of warfarin metabolism, leading to a supratherapeutic INR and increased risk of bleeding.
Fluconazole is a potent inhibitor of the cytochrome P450 enzyme CYP2C9, which is the primary enzyme responsible for metabolizing the more active S-isomer of warfarin. By inhibiting this enzyme, fluconazole decreases the clearance of warfarin, leading to higher plasma concentrations, an elevated International Normalized Ratio (INR), and a significantly increased risk of serious bleeding.
Question 3: Which of the following statements most accurately describes the mechanism of action of acyclovir in treating a herpes simplex virus (HSV) infection?
- It acts as a neuraminidase inhibitor, preventing the release of newly formed virions from the host cell.
- It directly inhibits the fusion of the viral envelope with the host cell membrane.
- It prevents the uncoating of the viral genome after it enters the host cell.
- It is a guanosine analog that, after phosphorylation by viral thymidine kinase, inhibits viral DNA polymerase and causes chain termination. (Correct answer)
Correct answer: It is a guanosine analog that, after phosphorylation by viral thymidine kinase, inhibits viral DNA polymerase and causes chain termination.
Acyclovir is a prodrug that is selectively activated in cells infected with herpes viruses. The viral enzyme thymidine kinase monophosphorylates acyclovir. Host cell kinases then convert it to acyclovir triphosphate, which competitively inhibits the viral DNA polymerase and is also incorporated into the growing viral DNA chain, causing chain termination. This specificity for the viral enzyme accounts for its selective toxicity.
Question 4: A patient undergoing chemotherapy with a regimen that includes cisplatin is closely monitored for adverse effects. Which of the following is a well-known and often dose-limiting toxicity of this agent?
- Cardiotoxicity
- Nephrotoxicity (Correct answer)
- Pulmonary fibrosis
- Hemorrhagic cystitis
Correct answer: Nephrotoxicity
Cisplatin is a heavy metal compound notorious for causing significant, cumulative, and dose-limiting nephrotoxicity. It accumulates in the renal tubules, leading to direct cellular damage, acute kidney injury, and electrolyte wasting. Management includes aggressive pre- and post-treatment hydration to mitigate this effect. Cardiotoxicity is associated with doxorubicin, pulmonary fibrosis with bleomycin, and hemorrhagic cystitis with cyclophosphamide.
Question 5: A patient is admitted with a severe skin and soft tissue infection. Cultures grow Staphylococcus aureus that is resistant to oxacillin. This resistance is conferred by the mecA gene. What is the primary mechanism of resistance in this organism?
- Increased production of beta-lactamase enzymes that hydrolyze the antibiotic.
- Efflux pumps that actively transport the antibiotic out of the bacterial cell.
- Alteration of the drug's target site, the penicillin-binding protein (PBP). (Correct answer)
- Modification of the antibiotic molecule by bacterial enzymes.
Correct answer: Alteration of the drug's target site, the penicillin-binding protein (PBP).
Methicillin-resistant Staphylococcus aureus (MRSA) resistance is primarily due to the acquisition of the mecA gene. This gene encodes for a novel penicillin-binding protein, PBP2a. PBP2a has a very low affinity for all beta-lactam antibiotics, allowing the bacteria to continue synthesizing its cell wall even in the presence of these drugs, rendering them ineffective.
Question 6: Metronidazole is a unique antimicrobial agent prescribed for a variety of infections. Its spectrum of activity is primarily limited to which of the following classes of organisms?
- Aerobic gram-positive cocci
- Atypical bacteria (e.g., Mycoplasma, Chlamydia)
- Fungi and yeasts
- Anaerobic bacteria and certain protozoa (Correct answer)
Correct answer: Anaerobic bacteria and certain protozoa
Metronidazole is a prodrug that requires reductive activation of its nitro group, a process that can only occur in the low-redox-potential environment found within anaerobic organisms. Therefore, its spectrum is limited to obligate anaerobic bacteria (e.g., Bacteroides, Clostridium) and certain protozoa (e.g., Trichomonas, Giardia, Entamoeba). It has no clinically significant activity against aerobic bacteria.
Pharmacology Antimicrobial and Chemotherapeutic Agents Questions and Answers 1
A patient is prescribed penicillin for a gram-positive bacterial infection.
What is the primary mechanism by which this antibiotic exerts its bactericidal effect?