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Chapter 10 Antimicrobial Drugs: Mechanism of Action Cefazolin is often given prior to surgery to prevent postsurgical wound infections. Which one of the following best describes the mode of action of cefazolin?

  1. Cefazolin is often given prior to surgery to prevent postsurgical

wound infections. Which one of the following best describes the

mode of action of cefazolin?

(A) It acts as an electron sink depriving the bacteria of reducing


(B) It binds to the 30S ribosome and inhibits bacterial protein


(C) It inhibits transcription of bacterial mRNA.

(D) It inhibits transpeptidases needed to synthesize peptidoglycan.

(E) It inhibits folic acid synthesis needed to act as a methyl donor.

  1. Which one of the following drugs inhibits bacterial nucleic acid

synthesis by blocking the production of tetrahydrofolic acid?

(A) Ceftriaxone

(B) Erythromycin

(C) Metronidazole

(D) Rifampin

(E) Trimethoprim

  1. Regarding both penicillins and aminoglycosides, which one of

the following is the most accurate?

(A) Both act at the level of the cell wall.

(B) Both are bactericidal drugs.

(C) Both require an intact -lactam ring for their activity.

(D) Both should not be given to children under the age of 8 years

because damage to cartilage can occur.

(E) They should not be given together because they are


  1. Listed below are drug combinations that are used to treat certain

infections. Which one of the following is a combination in which

both drugs act to inhibit the same metabolic pathway?

(A) Amphotericin and flucytosine

(B) Isoniazid and rifampin

(C) Penicillin G and gentamicin

(D) Sulfonamide and trimethoprim

  1. Regarding penicillins and cephalosporins, which one of the following

is the most accurate?

(A) Cleavage of the -lactam ring will inactivate penicillins but

not cephalosporins.

(B) Penicillins act by inhibiting transpeptidases but cephalosporins

do not.

(C) Penicillins and cephalosporins are both bactericidal drugs.

(D) Penicillins and cephalosporins are active against gram-positive

cocci but not against gram-negative rods.

(E) Renal tubule damage is an important adverse effect caused by

both penicillins and cephalosporins.

  1. Regarding antimicrobial drugs that act by inhibiting nucleic acid

synthesis in bacteria, which one of the following is the most accurate?

(A) Ciprofloxacin inhibits RNA polymerase by acting as a nucleic

acid analogue.

(B) Rifampin inhibits the synthesis of messenger RNA.

(C) Sulfonamides inhibit DNA synthesis by chain termination of

the elongating strand.

(D) Trimethoprim inhibits DNA polymerase by preventing the

unwinding of double-stranded DNA.

  1. Regarding aminoglycosides and tetracyclines, which one of the

following is the most accurate?

(A) Both classes of drugs are bactericidal.

(B) Both classes of drugs inhibit protein synthesis by binding to

the 30S ribosomal subunit.

(C) Both classes of drugs inhibit peptidyl transferase, the enzyme

that synthesizes the peptide bond.

(D) Both classes of drugs must be acetylated within human cells

to form the active antibacterial compound.

(E) Both classes of drugs cause brown staining of teeth when

administered to young children.

  1. The selective toxicity of antifungal drugs, such as amphotericin B

and itraconazole, is based on the presence in fungi of which one

of the following?

(A) 30S ribosomal subunit

(B) Dihydrofolate reductase

(C) DNA gyrase

(D) Ergosterol

(E) Mycolic acid

  1. The next three questions ask about the adverse effects of antibiotics,

which are an important consideration when deciding

which antibiotic to prescribe. Which antibiotic causes significant

neurotoxicity and must be taken in conjunction with pyridoxine

(vitamin B6) to prevent these neurologic complications?

(A) Amoxicillin

(B) Ceftriaxone

(C) Isoniazid

(D) Rifampin

(E) Vancomycin

  1. Of the following antibiotics, which one causes the most phototoxicity

(rash when exposed to sunlight)?

(A) Amphotericin B

(B) Ciprofloxacin

(C) Gentamicin

(D) Metronidazole

(E) Sulfamethoxazole

  1. Which of the following antibiotics causes “red man” syndrome?

(A) Azithromycin

(B) Doxycycline

(C) Gentamicin

(D) Sulfamethoxazole

(E) Vancomycin

✅ Answers (1)

Private answer
  1. (D)

Cefazolin inhibits cell wall biosynthesis by binding Penicillin binding proteins which stops peptidoglycan synthesis. Penicillin binding proteins are bacterial proteins that help to catalyze the last stages of peptidoglycan synthesis, which is needed to maintain the cell wall.



  1. (E)

Trimethoprim binds to dihydrofolate reductase and inhibits the reduction of dihydrofolic acid (DHF) to tetrahydrofolic acid (THF). THF is an essential precursor in the thymidine synthesis pathway and interference with this pathway inhibits bacterial DNA synthesis.




  1. (B)

The aminoglycosides are rapid acting bactericidal drugs, with their killing effect depending on their concentration. They also have residual bactericidal activity that is present after the serum concentration has fallen below the minimum inhibitory concentration.

Penicillin kills susceptible bacteria by specifically inhibiting the transpeptidase that catalyzes the final step in cell wall biosynthesis, the cross-linking of peptidoglycan.



  1. (D)

sulfamethoxazole and trimethoprim. It is used to treat a wide variety of bacterial infections (such as middle ear, urine, respiratory, and intestinal infections). It is also used to prevent and treat a certain type of pneumonia (pneumocystis-type).



  1. (C)

Cephalosporins are bactericidal and have the same mode of action as other β-lactam antibiotics (such as penicillins), but are less susceptible to β-lactamases. Cephalosporins disrupt the synthesis of the peptidoglycan layer forming the bacterial cell wall




  1. (B)

These in vivo results are consistent with the mode of rifampicin action determined from in vitro studies; rifampicin prevents initiations of RNA polymerase on deoxyribonucleic acid, but not its propagation, by binding the enzyme essentially irreversibly.




  1. (B)

Aminoglycosides inhibit protein synthesis in bacteria by binding irreversibly to the 30S ribosomal subunit. This inhibits transfer of aminoacyl-tRNA to the peptidyl site, causing premature termination of the peptide chain; it also increases the frequency of misreading of mRNA. Tetracyclines drugs are bacteriostatic and inhibit protein synthesis by blocking the association of tRNAs with the ribosome during translation.





  1. (D)

Amphotericin B, the azoles (fluconazole, itraconazole, ketoconazole, and voriconazole), and the echinocandins are the primary drugs used in systemic infections. They are selectively toxic to fungi because they interact with or inhibit the synthesis of ergosterol, a sterol unique to fungal cell membranes.



  1. (C)

ethambutol (eMB) and isoniazid (INH) are the first- line antituberculosis (anti-tB) drugs. However, their neurotoxicity could cause adverse effect and the patients with end-stage renal disease are especially vulnerable due to the reduction in renal drug clearance




  1. (E)

The reaction usually includes an eczema-type rash, which often occurs a few days after exposure. The rash can also spread to parts of the body that were not exposed to the sunPhototoxicity.




  1. (E)

Vancomycin can cause two types of hypersensitivity reactions, the red man syndrome and anaphylaxis. Red man syndrome is an infusion-related reaction peculiar to vancomycin. It typically consists of pruritus, an erythematous rash that involves the face, neck, and upper torso

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Answered on June 24, 2020 4:51 pm

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