Final answer:
The prenatal lab result to report is a positive syphilis serology due to its severe consequences for the fetus and the high effectiveness of prompt treatment.
Step-by-step explanation:
The prenatal laboratory finding that should be immediately reported to the provider is c) Positive syphilis serology. Syphilis is a sexually transmitted infection that can have severe consequences for both the mother and the fetus if left untreated, including fetal death. Prompt treatment with antibiotics, typically penicillin, is highly effective at preventing mother-to-fetus transmission. In contrast, elevated glucose levels and low hemoglobin may require monitoring or treatment but are not as immediately critical as an infectious disease like syphilis. While increased white blood cell count suggests an infection, it is commonly seen in pregnancy and does not hold the same level of urgency as a positive syphilis test.