In this task, you will be given sentences in which your task is to recognize the name of the organ of the body. An organ is a collection of tissues that structurally form a functional unit specialized to perform a particular function. Although there might be several correct answers, you need to write one of them.
One example: I have serious allergy in my left eye.
Solution is here: eye
Explanation: Eye is an organ which has been affected, hence should be tagged.

Now, solve this:  Tumor markers such as alpha - fetoprotein , human growth hormone , carcinoembryonic antigen , and placental alkaline phosphatase were negative .
Solution:
placental