Teacher: In this task, you are given a sentence. You are expected to write the name of the organism substances in the body mentioned in the input. Although there might be several correct answers, you need to write one of them.
Teacher: Now, understand the problem? If you are still confused, see the following example:
Antisperm antibodies were assessed in the serum samples of 73 men unable to ejaculate naturally and on the sperm cells of 13 of these men.
Solution: serum
Reason: Serum is a substance of the organism and needs to be tagged.

Now, solve this instance:  Fetal Down syndrome was diagnosed by karyotyping of amniotic fluid cells .
Student:
fluid