A 32-year-old primigravida at 18 weeks gestation presents for routine antenatal screening. Transabdominal ultrasound shows a single cystic lesion in the fetal abdomen with internal echoes, no peristalsis, and mild polyhydramnios. The lesion does not communicate with the bladder or bowel on real-time imaging. What is the investigation of choice to confirm the diagnosis and assess for associated anomalies?