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.

Let me give you an 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.
The answer to this example can be: serum
Here is why: Serum is a substance of the organism and needs to be tagged.

OK. solve this:
 Residential proximity to the plant and younger age ( up to 13 years old ) were the strongest predictors of an individual ' s serum 2 , 3 , 7 , 8 - TCDD level .
Answer:
serum