A 43-year-old nulliparous woman with an unremarkable personal and family history conceived spontaneously. First-trimester screening performed elsewhere was reported as normal. Non-invasive prenatal testing for trisomies 21, 18, and 13 was low risk (fetal fraction 13%). At 21 weeks and 3 days, an abnormal second-trimester anomaly scan prompted detailed fetal ultrasound assessment. The following findings were identiifed. What is the most likely diagnosis?