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Chapter 44: Chloramphenicol, Tetracyclines, Macrolides, Clindamycin, Streptogramins, & Linezolid

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A 4-year-old child is brought to the hospital after ingesting pills that a parent had used for bacterial dysentery when traveling outside the United States. The child has been vomiting for more than 24 h and has had diarrhea with green stools. She is now lethargic with an ashen color. Other signs and symptoms include hypothermia, hypotension, and abdominal distention. The drug most likely to be the cause of this problem is

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(A) Ampicillin

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(B) Azithromycin

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(C) Chloramphenicol

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(D) Doxycycline

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(E) Erythromycin

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Chloramphenicol is commonly used outside the United States for treatment of bacillary dysentery. The drug causes a dose-dependent (reversible) suppression of erythropoiesis. Although the gray baby syndrome was initially described in neonates, a similar syndrome has occurred with overdosage of chloramphenicol in older children and adults, especially those with hepatic dysfunction. The answer is C.

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The mechanism of antibacterial action of tetracycline involves

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(A) Antagonism of bacterial translocase activity

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(B) Binding to a component of the 50S ribosomal subunit

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(C) Inhibition of DNA-dependent RNA polymerase

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(D) Interference with binding of aminoacyl-tRNA to bacterial ribosomes

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(E) Selective inhibition of ribosomal peptidyl transferases

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Tetracyclines inhibit bacterial protein synthesis by interfering with the binding of aminoacyl-tRNA molecules to bacterial ribosomes. Peptidyl transferase is inhibited by chloramphenicol. The answer is D.

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Clarithromycin and erythromycin have very similar spectra of antimicrobial activity. The major advantage of clarithromycin is that it

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(A) Does not inhibit hepatic drug-metabolizing enzymes

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(B) Eradicates mycoplasmal infections in a single dose

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(C) Has greater activity against H pylori

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(D) Is active against methicillin-resistant strains of staphylococci

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(E) Is active against strains of streptococci that are resistant to erythromycin

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Clarithromycin can be administered less frequently than erythromycin, but it is not effective in single doses against susceptible organisms. Organisms resistant to erythromycin, including pneumococci and methicillin-resistant staphylococci, are also resistant to other macrolides. Drug interactions have occurred with clarithromycin through its ability to inhibit cytochrome P450. Clarithromycin is more active than erythromycin against M avium complex, T gondii, and H pylori. The answer is C.

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