Which agent is likely to be prescribed for a client with primary syphilis who is allergic to penicillin?

Tetracycline is a suitable alternative for treating primary syphilis in patients who are allergic to penicillin. This antibiotic belongs to a different class than penicillin and has demonstrated efficacy against Treponema pallidum, the bacterium responsible for syphilis. Tetracycline works by inhibiting protein synthesis in bacteria, thus preventing their growth and reproduction.

In cases where patients cannot receive penicillin due to an allergy, tetracycline serves as an effective second-line treatment option. It is especially important to recognize that tetracycline should not be given to pregnant women or children under eight years old due to potential adverse effects on bone and tooth development.

Other medications listed, such as Podophyllum resin, ceftriaxone, and acyclovir, do not address syphilis effectively. Podophyllum resin is used for treating condyloma acuminata (genital warts) and has no role in treating syphilis. Ceftriaxone, although it has some activity against Treponema pallidum, is not a standard alternative therapy for primary syphilis and is primarily indicated for other infections. Acyclovir is an antiviral medication used to treat herpes viruses, not bacterial infections like syph