The cell part that breaks down nutrients and foreign substances is the lysosome.

Here's why:

* Lysosomes are the "recycling centers" of the cell. They contain powerful enzymes that can break down various molecules, including:

* Nutrients: Lysosomes help digest food particles that have been engulfed by the cell.

* Foreign substances: They break down bacteria, viruses, and other harmful invaders.

* Worn-out cell parts: Lysosomes also help remove old or damaged organelles.

Let me know if you'd like to know more about lysosomes or other cell parts!