Annular rash on a newborn

A full-term, healthy female newborn was delivered via cesarean section because the labor did not adequately progress. The mother, age 33 years and of Asian ancestry, had a significant medical and obstetrical history: chronic hepatitis B carrier without cirrhosis, cutaneous lupus erythematosus (positive anti-Ro and anti-La antibodies), and a positive group B streptococcal recto-vaginal culture at 35 weeks' gestation. The mother received 4 doses of intravenous ampicillin during labor. The infant's initial hospital course was complicated by a transient and otherwise asymptomatic bradycardia. An electrocardiogram (ECG) confirmed a heart rate of 96 with normal interval parameters, but there were changes suggestive of left ventricular hypertrophy. An echocardiogram was normal. Follow-up office visits for common newborn feeding problems demonstrated consistent weight gain and normal vital signs, including heart rate and facial milia. However, by age 4 weeks an erythematous eruption extending from the frontal scalp and forehead to the cheek area had developed. What is the differential diagnosis? What tests should be done to make the diagnosis?