Fetal Trisomy 18 Choroid Plexus Cyst

A 41-year-old G3P2 at 20 weeks 5 days gestation is referred for detailed anomaly scan after first-trimester combined screening returns high-risk for trisomy 18 (1 in 50). On ultrasound, the fetus shows severe symmetrical intrauterine growth restriction, oligohydramnios, strawberry-shaped skull with flattened occiput, micrognathia, perimembranous VSD with aortic override, omphalocele, and persistently clenched fists with overlapping fingers. Both feet show rocker-bottom deformity. The structure marked **B** in the diagram—bilateral fetal choroid plexus cysts in the context of this constellation of anomalies—is most consistent with which chromosomal abnormality?