Answer is C (Karyotyping) The question specifically asks for an investigation before the next pregnancy. Out of the options provided only karyotyping is of value prior to pregnancy, the remaining three being of significance during the antenatal period for detection of fetal affection with Down's syndrome. Down's syndrome a trisomv 21 may arise from a. Non-disjunction of chromosome 21: b. Translocation (or mosaicism) : * Most common mode accounting for 95% cases * Accounts for remaining cases. * Risk of recurrence is one percent. * Risk of recurrence is high. * Parental chromosome study is not necessary but invasive prenatal diagnosis is indicated by either chorionic villous sampling or amniocentesis. * Parental chromosome study is indicated Karyotyping of Mrs. Annadurai has already been done and a translocation variety of Downs detected. Risk of recurrence however does not depend on mother's karyotype alone, but it also depends on the father's karyotype. Fathers kaiyotyping is therefore the test of choice prior to next pregnancy to determine the recurrent risk of having another child with Down's syndrome.