Differential Diagnoses for Skin Disease in Pregnancy

This same 42-year-old G2P1 female presents two weeks later at 30 weeks gestation after two weeks of treatment with fluocinonide ointment in a 1:3 ratio twice daily diluted with moisturizer diffusely over the trunk and extremities, loratadine 10 mg every morning, diphenhydramine 25 mg nightly and cephalexin 500 mg twice a day. Bacterial culture performed on the initial visit is negative for pathogens. What is the next diagnostic test that is recommended and what condition is now highest on the differential diagnosis?