- A disease causes bones become weak and brittle
- To become something is a kind of change
- Therefore, a disease causes bones to change

- A disease causes bones to change
- If something causes something else to change, then that something affects that something else
- So, a disease affects bones

- A disease affects bones
- Skeletal system is made of bones
- So, the disease affects parts of the skeletal system

- The disease affects parts of the skeletal system
- So, the disease affects the skeletal system

So, the disease affects the skeletal system
Which body system is most affected by a disease that causes the bones to become weak and brittle?
- Skeletal system is made of bones 
- If something causes something else to change, then that something affects that something else 
- A disease causes bones become weak and brittle 
- To become something is a kind of change