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Chapter 39: Penicillins, Cephalosporins, and Other β-Lactam Antibiotics

A 23-year-old woman develops a rash following her first dose of penicillin V. The most likely cause of the rash is an allergy to penicillin. What component of penicillin is the most frequent cause of an allergic reaction?

a. The intact penicillin molecule

b. The penicilloic acid moiety (major determinant moiety)

c. The 6-aminopenicillanic acid moiety

d. Other breakdown products (minor determinant moiety)

e. The side chain attached to the β-lactam ring through the amide linkage

Answer is b. A large percentage of immunoglobulin E-mediated reactions are to the major determinant moiety, but at least 25% of reactions are to other breakdown products which can be found in solutions of penicillin for administration.

A 48-year-old woman has an infection that cultures a S. aureus known to elaborate penicillinase. It is appropriate that she be treated with nafcillin because

a. blood concentrations of nafcillin are sufficiently high to kill all S. aureus.

b. nafcillin is more potent against S. aureus than penicillin G.

c. nafcillin is resistant to penicillinase.

d. nafcillin is active against methicillin-resistant microorganisms (MRSA).

e. nafcillin is a naturally occurring penicillin.

Answer is c. Nafcillin is a semisynthetic penicillin that is highly resistant to penicillinase and has proven effective against infections caused by penicillinase-producing strains of S. aureus. There is an increasing incidence of isolates of methicillin-resistant microorganisms (eg, MRSA) that are resistant to all penicillinase-resistant penicillins including nafcillin.

A 62-year-old man is being treated for gonorrhea with a third-generation cephalosporin, ceftriaxone. Ceftriaxone is the drug of choice for gonorrhea rather than a first- or second-generation cephalosporin because

a. this third-generation cephalosporin has a shorter plasma half-life than the first- or second-generation cephalosporins.

b. ceftriaxone has a half-life of about 8 hours making it effective as a single dose.

c. it has a much lower incidence of adverse reactions compared to other cephalosporins.

d. it is active against methicillin-resistant bacteria.

e. ceftriaxone is well absorbed after oral administration.

Answer is b. A single dose of ceftriaxone is effective in the treatment of urethral, cervical, rectal, or pharyngeal gonorrhea. A single dose is effective ...

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