A 3-day-old female infant, born at 39 weeks' gestation via spontaneous vaginal delivery, is undergoing evaluation in the neonatal intensive care unit. Her mother, a G2P1, experienced an unspecified febrile illness with flu-like symptoms during the first trimester, for which she received no specific treatment or detailed investigation due to limited access to care. The infant's vital signs are: temperature 37.2°C, heart rate 140 beats/min, respiratory rate 45 breaths/min, and blood pressure 65/45 mmHg. On physical examination, diffuse purpuric lesions resembling a "blueberry muffin" rash are noted across the trunk and extremities, along with significant hepatosplenomegaly extending 3 cm below the costal margins. Fundoscopic examination reveals focal retinal scarring and areas of active inflammation. Neurological assessment shows subtle hypotonia and a weak suck reflex. Given these findings, what is the most likely underlying congenital infection and the appropriate initial therapeutic intervention?