Final answer:
A pregnant woman's allergy to penicillin is the most critical information to gather if diagnosed with primary syphilis, due to the importance of treating with penicillin to prevent transmission to the fetus.
Step-by-step explanation:
The most important assessment data to gather for a pregnant woman diagnosed with primary syphilis would be A. Allergy to penicillin. Since penicillin is the antibiotic of choice with a 98 percent success rate at preventing mother-to-fetus transmission of syphilis, it is crucial to know whether the woman is allergic to it. If she is, desensitization is required before she can be treated with penicillin to mitigate the high risks associated with congenital syphilis, such as miscarriage, stillbirth, and severe neonatal health problems.
The focus must be on ensuring both the mother's and the fetus's safety by providing effective treatment while managing any potential allergies to the prescribed medication.