If an OBGYN wanted your opinion of treating a patient with syphilis who is allergic to penicillin, what would you advise?

Since penicillin is the drug of choice and the patient is allergic to it, you only have two choices--use an alternative drug or desensitize the patient to penicillin and give it as the drug of choice. The problem is that alternative drugs may not cross the placenta and prevent/treat infection of the unborn baby. Although this question assumes knowledge that crosses specialties, you can't go wrong by inviting appropriate consultation to help.