Paediatricsniche
A 4-month-old girl is referred because her head circumference has fallen from the 25th percentile at birth to below the 3rd percentile. She was born at 39 weeks weighing 2.4 kg. She was referred on newborn hearing screening in both ears, and diagnostic testing has confirmed bilateral sensorineural hearing loss. Her mother recalls a scattered rash of small purple-blue spots on the first day of life that faded over a week. She does not yet fix on faces and her head control is poor. There are no dysmorphic features, and her tone is increased. Which one of the following investigations is most likely to establish the cause?