Final answer:
In the case of a client with primary syphilis who is allergic to penicillin, a physician would likely prescribe tetracycline as an alternative treatment.
Step-by-step explanation:
If a client with primary syphilis is allergic to penicillin, the physician is likely to prescribe an alternative antibiotic. Tetracycline is one such alternative that can be used to treat syphilis in patients with penicillin allergy. Other possible antibiotics include doxycycline and macrolides, although these alternatives may require more careful monitoring and retesting for syphilis after treatment to ensure the bacterium is fully eradicated.