Final answer:
The first-line agent for treating recurrent GAS pharyngitis or as an alternative when a patient has had an anaphylactic reaction to penicillin is azithromycin (option b) , a macrolide antibiotic.
Step-by-step explanation:
If a patient has experienced an anaphylactic reaction to penicillin or treatment for recurrent Group A Streptococcus (GAS) pharyngitis is necessary, alternative antibiotics need to be considered.
In such cases, the first-line agent that can be used as an alternative to penicillin is azithromycin. Azithromycin is part of the macrolide class of antibiotics and functions by targeting bacterial ribosomes, which differs from penicillin's mechanism of targeting peptidoglycan synthesis.
Other alternatives include erythromycin, another macrolide, and cephalosporins, which may be used if the patient has not had a severe allergic reaction to the beta-lactam structure common to both cephalosporins and penicillins.