Which of the following is the investigation of choice in a pregnant lady at 17 weeks of pregnancy, with past history of delivering a baby with Down syndrome?

Because there is a past history of Down syndrome, a confirmatory test should be done. Amniocentesis and karyotyping is the best choice here. It is generally done around 16-18 weeks and gives confirmatory results. The triple marker is only a screening test and not a confirmatory test. USG can pick up soft tissue markers of Down syndrome, but it is not confirmatory and USG can be normal in a fetus with Down syndrome. If the same patient presents at 11-12 weeks, then the answer is CVS and karyotyping.