Approach to congenital infections Quiz: Ace Your Exams

A male infant delivered at 39 weeks via vaginal delivery is evaluated in the neonatal intensive care unit (NICU) for microcephaly and a rash. The pregnancy was uncomplicated. There was no use of recreational substances by the mother and no exposure to pets. Membranes ruptured spontaneously 6 hours prior to delivery, and delivery was uncomplicated. Apgar scores were 7 and 8 at one and five minutes, respectively. Temperature is 37°C (98.6°F), pulse is 140/min, respirations are 32/min, blood pressure is 67/38 mmHg, and oxygen saturation is 99% on room air. Weight is at the 5^th percentile, body length is at the 10^th percentile, and head circumference is at the 3^rd percentile. On physical examination, there is a generalized blueish petechial rash on the face and extremities. Cardiopulmonary and abdominal exams are normal. Fundoscopic exam reveals chorioretinitis. Brain imaging shows periventricular calcifications and ventriculomegaly. Which of the following tests is most likely to confirm the diagnosis?