If 50 out of 200 interphase nuclei have 3 signals in a prenatal aneusomy FISH analysis for chromosome 18, what does this indicate?

The presence of 3 signals in a prenatal aneuploidy FISH analysis for chromosome 18 indicates that there is likely an additional copy of chromosome 18 present in some of the cells analyzed, which is characteristic of trisomy 18. In a normal diploid cell, you would expect to see 2 signals representing the two copies of chromosome 18. When 50 out of 200 interphase nuclei show 3 signals, this suggests that a significant proportion of the nuclei are exhibiting the extra chromosome, supporting the diagnosis of trisomy 18.

While other options address different scenarios, none align with the evidence presented. A would suggest normal results where only two signals would be expected. C implies uncertainty; however, the number of signals observed provides clear information regarding the presence of additional chromosome material. D points to monosomy 18, which would result in fewer signals than expected, contrasting directly with the findings of extra signals. Therefore, the indication of 3 signals aligns with trisomy 18 or a structural anomaly.